Audio Evaluation — WER, MOS, UTMOS, MMAU, FAD, and the Open Leaderboards

> You cannot ship what you cannot measure. This lesson names the 2026 metrics for every audio task: ASR (WER, CER, RTFx), TTS (MOS, UTMOS, SECS, WER-on-ASR-round-trip), audio-language (MMAU, LongAudioBench), music (FAD, CLAP), and speaker (EER). Plus the leaderboards where you compare.

Type: Learn

Languages: Python

Prerequisites: Phase 6 · 04, 06, 07, 09, 10; Phase 2 · 09 (Model Evaluation)

Time: ~60 minutes

The Problem

Every audio task has multiple metrics, each measuring a different axis. Using the wrong metric is how you ship a model that looks great on your dashboard and terribly in production. The 2026 canonical list:

The Concept

ASR metrics

WER (Word Error Rate). (S + D + I) / N. Lowercase, strip punctuation, normalize numbers before scoring. Use jiwer or OpenAI's whisper_normalizer. < 5% = human-parity read speech.

CER (Character Error Rate). Same formula, character-level. Used for tone languages (Mandarin, Cantonese) where word segmentation is ambiguous.

RTFx (inverse real-time factor). Audio seconds processed per wall-clock second. Higher is better. Parakeet-TDT hits 3380×. Whisper-large-v3 is ~30×.

First-token latency. Wall-clock from audio input to first transcript token. Critical for streaming. Deepgram Nova-3: ~150 ms.

TTS metrics

MOS (Mean Opinion Score). 1-5 human rating. Gold standard but slow. Collect 20+ listeners per sample, 100+ samples per model.

UTMOS (2022-2026). Learned MOS predictor. Correlates ~0.9 with human MOS on standard benchmarks. F5-TTS: UTMOS 3.95; ground truth: 4.08.

SECS (Speaker Encoder Cosine Similarity). For voice cloning. ECAPA embedding cosine between reference and cloned output. > 0.75 = recognizable clone.

WER-on-ASR-round-trip. Run Whisper over TTS output, compute WER against the input text. Catches intelligibility regressions. 2026 SOTA: < 2% CER.

TTFA (time-to-first-audio). Wall-clock latency. Kokoro-82M: ~100 ms; F5-TTS: ~1 s.

Voice-cloning-specific

SECS + MOS + CER as a triple. Cloning that scores high SECS but low MOS means timbre-right-but-unnatural; the opposite means natural voice but wrong speaker.

Speaker verification

EER (Equal Error Rate). The threshold where False Accept Rate equals False Reject Rate. ECAPA on VoxCeleb1-O: 0.87%.

minDCF (min Detection Cost). Weighted cost at a chosen operating point (often FAR=0.01). More production-relevant than EER.

Diarization

DER (Diarization Error Rate). (FA + Miss + Confusion) / total_speaker_time. Missed speech + false-alarm speech + speaker-confusion, each as a fraction. AMI meetings: DER ~10-20% is realistic. pyannote 3.1 + Precision-2 commercial: <10% DER on well-recorded audio.

JER (Jaccard Error Rate). Alternative to DER, robust to short-segment bias.

Audio classification

Multi-label: mAP (mean Average Precision) over all classes. AudioSet: 0.548 mAP for BEATs-iter3.

Multi-class exclusive: top-1, top-5 accuracy. Speech Commands v2: 99.0% top-1 (Audio-MAE).

Imbalanced: macro F1 + per-class recall. Report per-class — aggregate accuracy hides which classes fail.

Music generation

FAD (Fréchet Audio Distance). Distance between VGGish-embedding distributions of real vs generated audio. MusicGen-small on MusicCaps: 4.5. MusicLM: 4.0. Lower better.

CLAP Score. Text-audio alignment score using CLAP embeddings. > 0.3 = reasonable alignment.

Listening panel MOS. Still the final word for consumer-grade music. Suno v5 ELO 1293 on TTS Arena (from paired human preferences).

Audio-language benchmarks

MMAU (Massive Multi-Audio Understanding). 10k audio-QA pairs.

MMAU-Pro. 1800 hard items, four categories: speech / sound / music / multi-audio. Random chance 25% on 4-way. Gemini 2.5 Pro overall ~60%; multi-audio ~22% across all models.

LongAudioBench. Multi-minute clips with semantic queries. Audio Flamingo Next beats Gemini 2.5 Pro.

AudioCaps / Clotho. Captioning benchmarks. SPICE, CIDEr, FENSE metrics.

Streaming speech-to-speech

Latency P50 / P95 / P99. Wall-clock from end-of-user-speech to first audible response. Moshi: 200 ms; GPT-4o Realtime: 300 ms.

WER / MOS on the output.

Barge-in responsiveness. Time from user interrupt to assistant mute. Target < 150 ms.

The 2026 leaderboards

Build It

Step 1: WER with normalization

from jiwer import wer, Compose, ToLowerCase, RemovePunctuation, Strip

transform = Compose([ToLowerCase(), RemovePunctuation(), Strip()])
score = wer(
    truth="Please turn on the lights.",
    hypothesis="please turn on the light",
    truth_transform=transform,
    hypothesis_transform=transform,
)
# ~0.17

Step 2: TTS round-trip WER

def ttr_wer(tts_model, asr_model, texts):
    errors = []
    for txt in texts:
        audio = tts_model.synthesize(txt)
        recog = asr_model.transcribe(audio)
        errors.append(wer(truth=txt, hypothesis=recog))
    return sum(errors) / len(errors)

Step 3: SECS for voice cloning

from speechbrain.inference.speaker import EncoderClassifier
sv = EncoderClassifier.from_hparams("speechbrain/spkrec-ecapa-voxceleb")

emb_ref = sv.encode_batch(load_wav("reference.wav"))
emb_clone = sv.encode_batch(load_wav("cloned.wav"))
secs = torch.nn.functional.cosine_similarity(emb_ref, emb_clone, dim=-1).item()

Step 4: FAD for music generation

from frechet_audio_distance import FrechetAudioDistance
fad = FrechetAudioDistance()
score = fad.get_fad_score("generated_folder/", "reference_folder/")

Step 5: EER for speaker verification (same code as Lesson 6)

def eer(same_scores, diff_scores):
    thresholds = sorted(set(same_scores + diff_scores))
    best = (1.0, 0.0)
    for t in thresholds:
        far = sum(1 for s in diff_scores if s >= t) / len(diff_scores)
        frr = sum(1 for s in same_scores if s < t) / len(same_scores)
        if abs(far - frr) < best[0]:
            best = (abs(far - frr), (far + frr) / 2)
    return best[1]

Use It

Pair every deploy with a fixed eval harness that runs on every model update. Three cardinal rules:

1. Normalize before scoring. Lowercase, punctuation-strip, number-expand. Report the normalization rule.
2. Report distributions, not averages. P50/P95/P99 for latency. Per-class recall for classification. Per-category for MMAU.
3. Run one canonical public benchmark. Even if your production data differs, reporting on Open ASR / TTS Arena / MMAU lets reviewers compare apples-to-apples.

Pitfalls

• UTMOS extrapolation. Trained on VCTK-style clean speech; scores noisy / cloned / emotional audio poorly.
• MOS panel bias. 20 Amazon Mechanical Turk workers ≠ 20 target users. Pay for a domain panel if stakes are high.
• FAD depends on reference set. Compare against the same reference distribution across models.
• Aggregate WER. A 5% WER overall can hide 30% WER on accented speech. Report by demographic slice.
• Public benchmark saturation. Most frontier models are near the ceiling on standard benchmarks. Build an in-house held-out set that reflects your traffic.

Ship It

Save as outputs/skill-audio-evaluator.md. Pick metrics, benchmarks, and reporting format for any audio model release.

Exercises

1. Easy. Run code/main.py. Compute WER / CER / EER / SECS / FAD-ish / MMAU-ish on toy inputs.
2. Medium. Build a TTS round-trip WER harness. Run your Kokoro or F5-TTS output through Whisper. Compute WER over 50 prompts. Flag prompts with WER > 10%.
3. Hard. Score your Lesson 10 LALM choice on MMAU-Pro speech + multi-audio subsets (50 items each). Report per-category accuracy and compare with the published number.

Key Terms

Further Reading

• jiwer — WER/CER library with normalization utilities.
• UTMOS (Saeki et al. 2022) — learned MOS predictor.
• Fréchet Audio Distance (Kilgour et al. 2019) — the music-gen standard.
• Open ASR Leaderboard — 2026 live rankings.
• TTS Arena — human-vote TTS leaderboard.
• MMAU-Pro benchmark — LALM reasoning leaderboard.
• HEAR benchmark — audio SSL benchmarks.