I found that after upgrading to the latest version of PMS, the PMS server will no longer list .mp4 files with 5.1ch AAC audio to PS3 renderer.
I checked the PS3.conf and found that the reason is the following line:
Supported = f:mp4 v:mp4|h264 a:aac m:video/mp4 n:2
After changing the n:2 into n:6, these files can be displayed and played (without transcoding) normally with PS3.
So I wonder why the default PS3.conf is configured like this?