Can't hear bass and drum in part of .wav playback

I apologize that I have a bit of a complicated question. I am composing a score for orchestra and two voices. In general, when I save to a .wav file and play the .wav file I can hear the bass (Contrebasses) and bass drum. So I know the problem isn't the speakers on the PC. At one point in the score I have some notes in the violins, violas and cellos that are held for a long time (304 measures at a slow tempo). At the same time the celesta, trumpet, bass drums and basses are playing. I know this is a weird combination, but it is because of the text. The problem is that in this passage the basses and bass drum are not audible. The .wav file synthesis is non-real-time synthesis, so these parts should be computed.

I wonder whether there is another thing I could try: saving to an xml file, thenn, is there a way to generate a .wav file from an xml file, and would that allow me to hear these two instruments?

Any suggestions on why these two instruments are inaudible during this passage and how to make the playback work would be most appreciated.