There are many possible reasons for one-way audio, for example routing problems, incorrect classification of traffic intended for the VPN and so on. It is possible that the inspect H323 feature could break something too but it would mostly depend on whether your are intending to NAT the traffic between the two offices (normally inter-office traffic is not NATed).
If it is not a major change for your organisation to change inspection rules then you could quickly rule it in or out by disabling but I would only disable the specific application, or in other words:
inspect h323 h225 <<<<<<<<<<<<<<< removing this line
inspect h323 ras <<<<<<<<<<<<<<< removing this line
All of this assumes you are using H.323 for audio of course.
Another approach is to start troubleshooting at the beginning, something like below:
1) In which direction is the audio traffic failing?
2) Capture the audio RTP stream, unencrypted , nearest the source and confirm the destination IP address. Is it correct?
3) confirm the unencrypted traffic is permitted by ACLs on the Firewall.
4) On reaching the firewall confirm the VPN process is correctly classifiying the unencrypted traffic and encrypting it / tunneling through the VPN
5) confirm the traffic is correctly decrypting and egressing at the far.
And of course all of this assumes that basic checks have been done, such as testing the far-end microphones, far-end mute etc. Apologies if you have already tested this but if I had a dollar for everytime the really basic stuff gets overlooked I'd be a wealthy man :)
Routing would be a good bet. Remember, you have to be able to initiate a path in both directions, both to the telephony device and all primary/secondary call managers, but ALSO to the voice gateways. Since you can initiate the call, verify what voice gateways will be assigned for that site, and that the end devices AND gateways both have correct routing to device AND to assigned loopback addresses if they are used.
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