When synching audio files on a MIDI sequencer/digital audio software to external timecode, most software supports only "freewheel" style sync between the audio on hard disk and the external timecode. That is, the timecode triggers hard disk audio playback, but does not actually lock it to external timing control. In this situation, you can avoid "drift" between the timecode and audio tracks, and achieve reasonably tight sync of the two by "breaking" the audio files down into 8 or 16 bar segments (best length depends on tempo), then laying these segments exactly on the downbeats of the appropriate measures. Assuming the original tracks were recorded with good timing, it should be easy to line these up. Plus, most sequencers will allow you to "slide" the tracks back and forth in time to further "tighten" the tracks up.