I recommend you post this to the Cisco Support Community for more information and feedback from technical experts.
I hope this helps.
Moderator for Cisco Customer Communities
Thanks. I already post that on Cisco Support Community and got an answer.
The issue was my pstn gateway loopback was not advertise on site to site vpn tunnel routes, it was not reachable as point to point from tunnel, after fixing that, audio for VOIP external call over site to site vpn tunnel works fine.