Do any other tracks (Midi or RealDrums) lose volume?

I would check two things:

1. Bar settings. Press F5 and hit the 'Reset All Bars' button. If this solves the problem, somewhere in the song is a Bar Setting reducing volume. Be careful as you will lose all of your bar settings with this option so you may make a backup before doing this.

2. If all tracks are losing volume, you may have the Fadeout ending enabled. Find this is the Song Settings (Ctrl N). If set to anything other than 0, the tracks will fade.

R