why does the audio need rendering?
if it is an mp3 or AAC file, you're asking for trouble
best to convert to AIFF 16bit 48k.
if you've got a lot of tracks, or have used some audio filters,
go to your user prefs (option Q) and change the number of real time audio tracks.
(this also allocates more memory for audio filters)
default is 8, you can easily go up to 24.
cheers,
nick