Deepgram

Whisper is OpenAI's open-source, MIT-licensed speech-to-text model trained on 680,000 hours of audio -- you can download it and run transcription fully offline and free on your own hardware. It supports ~99 languages plus translation to English and is remarkably robust to accents and noise. If you don't want to run GPUs, OpenAI's hosted transcription API runs Whisper (and newer gpt-4o-transcribe models) at roughly $0.006/min. It has no built-in speaker diarization and the core repo updates infrequently, but the surrounding ecosystem (whisper.cpp, faster-whisper, WhisperX) is enormous.

Cartesia builds real-time-first voice models -- its Sonic TTS and Ink STT rank #1 on Artificial Analysis speech leaderboards for combined quality and speed. Built on state-space (Mamba-style) architectures for ultra-low latency, it's purpose-made for voice agents and powers platforms like Retell. One developer API covers TTS, STT, and voice agents, with a genuinely usable free tier (20K credits/mo) and paid plans from $5/mo, plus cloud, on-prem, and on-device deployment. The main friction is an abstract credit model and promo pricing that muddies the long-term cost.

AssemblyAI is a developer-first speech-to-text platform built for production voice AI, with industry-leading accuracy from its Universal model family. Beyond transcription it offers a Speech Understanding API (summarization, sentiment, PII redaction), a Voice Agent API, and an LLM Gateway (formerly LeMUR) that runs LLMs over transcripts at passthrough pricing. Pricing is transparent pay-as-you-go -- pre-recorded from $0.15/hr, realtime from $0.15/hr -- with an unusually generous free tier (185 hours pre-recorded, 333 hours streaming, no card). It's a builder's tool, not a consumer app, and competes head-to-head with Deepgram.

Industry-leading AI voice platform for text-to-speech, voice cloning, and multilingual dubbing. Produces the most natural-sounding synthetic speech with instant cloning from short samples. Used by podcasters, game devs, and SaaS companies building voice features. Robust API for easy integration.

Stable Audio is Stability AI's music and sound-effects generator, and the only major player offering open-weight music models trained on fully licensed data. The hosted app (running Stable Audio 2.5) has tiers from free to $89.99/mo, while the Stable Audio 3.0 Small and Medium models released in May 2026 are open weights on Hugging Face, free for commercial use under $1M revenue. That means you can self-host, own your outputs, and generate variable-length tracks up to six minutes. The hosted free tier is thin (10 generations, 30-second crop, non-commercial), but the open-weight option is genuinely unique.

Resemble AI started as a voice-cloning and text-to-speech platform and has expanded into 'generative AI security' -- it generates voices, watermarks them, and detects deepfakes across audio, image, and video. Pricing is transparent pay-as-you-go (TTS around $0.0005/sec, ~$1.80/hr) with credits that never expire, plus custom enterprise. There's no ongoing free tier, just initial credits to start. It open-sourced its Chatterbox TTS model (popular on Hugging Face), and its real differentiator is provenance and deepfake defense, not the cheapest narration.