I haven't seen anyone ask about this issue. When I started up the game I noticed that the music was way to loud. When I went to the sound options menu, I tried to lower the volume. To my surprise the music did not lower in the slightest. So I tried some solutions and found one.

In the main Tomb Raider 1 folder, open up the dosboxTR_single.conf file. In there you will see this text.

mount C ".."
imgmount d "..\game.dat" -t iso -fs iso
c:
cd TOMBRAID
cls
tomb.exe
exit

Now in there you should put this line of text.

mixer cdaudio 5:5

This is what the text should look like now.

mount C ".."
imgmount d "..\game.cue" -t iso -fs iso
c:
mixer cdaudio 5:5
cd TOMBRAID
cls
tomb.exe
exit

Now save the changes.

What this does is change the volume of the cd audio. 5:5 is the volume levels for the left and right audio. I kept it 5:5 because I found to be the right volume level for the music. You can change it to whatever you want from 1:1 to 100:100. Just make sure both sides have the same number.