You can upload 25000 mp3s to Google Music for free — no subscription and no ads. Then just ask the speaker to play them or cast from your phone.

Google home works as an audio Chromecast target, so you can play MP3 from any app that supports Chromecast audio playback.

Personally I use Plex.

You could AirPlay it to the HomePod. It also supports FLAC.

google home: bluetooth from your device or upload from your device to google play music and use that