May contain traces of nut
It's not that the HDR-FOX "uses" AAC; DVB-T2 broadcast streams are H.264/AAC. The PVR simply records what it receives (as do all PVRs).HDR-FOX uses the AAC audio codec
You are correct: DVB-S2 is H.264/AC3 (according to my brief google). The HiDef audio stream is not cross-compatible.
Note it is not unrealistic to consider converting the audio stream using the HDR-FOX and the appropriate ffmpeg command, pre-export or post-import (the Foxsat is less powerful). I'm not sure anyone has tried an AAC/AC3 conversion, but MP2-MP3 conversion is approximately real-time.I use FLV MP3 converter to create the ac3 file and tsmuxergui to remux the video and audio for AV2HDR. Note AV2HDR has problems with the latest version of tsamuxer - look for version 1:10:06.