I use audio to TV via HDMI cable.
I have the following issues when selecting an AC3 audio track:
1) when AC3 downmix OFF is set, the audio level is the same as the MPEG track, but when switching channels, the speakers sound popping or cracking!
2) when AC3 downmix ON is set, the sound is much lower than the MPEG track and channel switching is free of disturbing sounds