Ad sources: whew! At least now you'll backup
Yeah, the playlist way works, but it is quite slow (to do it by mouse, not program response...). And on this computer, using non-VMR renderer and latest stable ffdshow (official milan's build), it does NOT seek in video after I load the .mp3 in playlist. It works from File>Open though (i.e. I load the .avi, open playlist, select .mp3 and it loads almost - not exactly, but close - synchronized audio + video. But then when I seek, video stops for ~5 seconds and then continues as if nothing happened, while audio seeks correctly; resulting in total desynchronization).
Bottom line: probably the only really compatible way is to throw the graph away and build a new one from scratch - it takes ~3 times as much time (about 1.5sec here), but works correctly.
Also it'd be nice if you could implement some weird keyboard shortcut for "next audio track", as in DVobSub (so that it's scriptable for remote control...)