I'd like to know how conformance of decoders has to be tested. I think that there are some reference tracks, but where to got them?

The compliance testing bitstreams can be found here:

ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg1/compliance/ ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg2/compliance/

The compliance tests themselves are defined in ISO/IEC 11172-4 and 13818-4.

Cheers, -rob