A few things: By editing the AC3 audio in AUdacity, AUdacity can only reencode, so you’ll take a quality hit. For lossless editing, you can try mkvtoolnix or delaycut to add silence & cut etc.
AUdacity cannot make DTS. try Surcode or DTS master audio suite.
Thanks, i’ll try the other softwares. With audacity and already compressed files used it’s not an issue (well… I might be wrong about that): for instance, a 448kbps file can be saved with the same quality.