Thanks for the replies so far! I thought there is used some way without alsa because it was disabled on purpose on the build instructions http://www.raspbian.org/RaspbianXBMC
I indeed tested the existing "boost volume on downmix" option (now again) and it has no effect though.
However I experienced that sometimes (rare) some audio (i guess one of the downmixed channels) in a movie (ac3) is remarkable louder (~better) than the rest. To be more precise i guess there are basically two problems:
1. The maximum possible generic sound output (e.g. mp3 audio lines) is a bit low (but quite acceptable - hey it's beta!)
2. The ac3 (oh and it's dts too) stereo downmix to analog isn't working properly = very low volume (which has a possible connection to nr. 1)
I don't think it's power related because I got a 1000mA supply and a (active) powered usb hub and I don't think the cpu is overloaded because audio and video are performing without stuttering (video after few seconds audio right from the start) and responsive.
Is it a good idea to search for the responsible code and raise some figures? I'm not that bad in c++ but i guess audio programming is a thing of its own, isn't it? I read sth about a standard setting of the raspi onboard card reader clock which was set too low for normal/expected speeds. Could sth like this be the case with the analog audio jack, too?