Are you working with imported audio, RealTracks, or MIDI in particular on this track? You mentioned audio but as you also mentioned the melody track which is often used for MIDI melodies (either recording or generation) we simply want to be sure what exactly you have loaded onto the melody track. Also, what kind of plugins are you trying to use?

You also mentioned channels. Channel 4 is typically the melody within Band-in-a-Box, as shown in the attached screenshot. There's actually a volume adjust for it that I've also indicated for the MIDI channel. By default it is usually set to 90, but you can check under Options | Preferences | Channels to see if you have it currently set lower or try increasing it if volume for MIDI melody is the issue.

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channel4melodyvolume.png (13.84 KB, 9 downloads)

Cheers,
Ember