Just to confirm, this morning I switched firmware to 1.3b8 and
incoming audio on incoming calls stopped working. I switched back to
1.232 and everything started working again as expected.

Fwiw, I checked firewall logs and didn't see anything related being blocked.

Any ideas / suggestions on how to track this down?

Someone suggested offlist (thanks!):
> Try disabeling portmapping on the outbound NAT.

I've seen that option on pfsense (and tried it to no avail) but when I
poked around the m0n0wall 1.3b8 I couldn't see that setting.

thx, michael