There shouldn't be any audio drops. At least MP does not drop audio frames. It drops video frames when video is falling behind audio. It does nothing with audio.However, every now and then (maybe once every evening), the audio drops out heavily and if this starts, it remains until MediaPortal or even the htpc is restarted!
Can you try some other Codec for audio passthru to you receiver? I had similar issues with some MPA builds under Vista and Win7 and choppy audio playback.
What also could help is enlarging the buffer of the Halli splitter and maybe having MP run at a higher thread priority. At least for testing if this helps in your case.