The current plan for Phonos is to use a proprietary Google API when being deployed to Wikimedia sites. There are some free software options for the same functionality, but are supposedly not up to par. This task outlines and tracks those deficiencies, so we can file upstream bugs and switch when a free option can replace Google.
An analysis was done in T307624: [16 hours] Investigate: Options of TTS engines with some notes on https://meta.wikimedia.org/wiki/Community_Wishlist_Survey_2022/Reading/IPA_audio_renderer/TTS_investigation