Honestly if I were you I would always disable the TV speakers and just have your speaker system playing (That's if you already have) because the sound usually does not have the same clarity and sometimes the sound can come out of sync between the two speakers.
For the delay 2.1 speaker systems tend not to need any delay but this can vary from tv to tv so experiment until the audio and picture match up.
PCM is transmitted better than Dolby Digital because Dolby digital is transmitted up to 480kbps and PCM can be transmitted up to 24mbps and also PCM Is uncompressed compared to Dolby digital resulting in PCM having better, clearer sound but yet again it also depends on your personal view.
Hope this helps