Another way:

Download, from GOG the mP3 soundtrack, put the tracks in a folder,start the musicin the folder usiung Windows media or whatever your favorite MP3 playing program is, go into the game, and then in options turn off the music (the 8 track player). You should be able to adjust the music volume in windows media or your preferred MP3 Program.
The songs play in a random pattern anyway,it just background and not cued to gameplay,so you lose nothing.

This is a belated update, but I'm adding that using the command "mixer cdaudio <l>:<r>" may be insufficient with the last SVN revision of DOSBox. Setting "sbmixer=false" in the corresponding DOSBox .conf file may do the job (thanks Qbix!), although you won't be able to control the sound volume in-game.

Sound volume can still be controlled before launch of RR with the "mixer" command, or alternatively by changing the volume for the DOSBox app as a whole.

Reason for the need to use "sbmixer=false" is that otherwise, DOS apps may control the CD audio volume as of the last SVN revision. In the case of of RR, though, it seems to always reset the volume to 100%, with no way to let you change this in-game.

To compare, Blood (OUWB v1.21) also resets the volume to 100%, but it's at least controllable from the corresponding in-game menu.