A recent comment on reddit by a user who seems to be a Purism employee sounds like phone call audio may be working now: https://www.reddit.com/r/Purism/comments/e8y36v/how_to_follow_librem_5_development_via_issues_at/fag2x8v

According to that comment, the following two actions may help:

apt install linux-image-librem5

Adjust speaker control in ALSA in alsamixer/amixer (needed once per boot)

It would be interesting if someone having a Birch phone could try this.