I’m not very familiar with the ‘new’ GCP firewall, but please try the following:
In Asterisk SIP Settings, General tab, NAT settings, click Detect Network Settings. External Address should populate with your 34.92.xx.xxx address and Local Networks should fill in your 10.x.x.x LAN address. On the chan_pjsip tab, leave External IP Address and Local network blank. Submit. Restart Asterisk.
From a pjsip extension (assuming it registers ok), make a test call to *43 (echo test). If there is no audio, at the Asterisk command prompt, type
pjsip set logger on
and make another test call.
Paste the Asterisk log for the call (which will now include a SIP trace) at pastebin.freepbx.org and post the link here.