For some, having music playing while trying to fight is annoying and it can detract from the combat experience. Music in the Red Faction custom levels (or maps) is a funny thing. Your control over it is dependent upon how the level was made. In most cases the music volume can be controlled. When you first open Red Faction click on Options -> Audio and adjust the music volume there. In some levels, the music volume cannot be adjusted.

Also, some levels have additional music files that are bundled with the map vpp file that you may find after extracting the contents of the zip file. In those cases, you can move the music vpp file or delete it out of the Red Faction\user_maps\multi folder. Just go to the multi folder and look through the vpp files. You may find some named [level name]music.vpp. You can safely remove these files.

Click to enlarge. Shown is the music vpp for the level "Scar Tissue".

For example, the level named DM_mf_All_Around_The_World is very popular among deathmatch players. However, if you download it from one of the more common sources such as redfaction.levels4you.com or redfactionview.net, it comes bundled with a music file named mf_AATW_music.vpp. The volume level for this song cannot be adjusted and it's quite loud. (It's also almost ten minutes long.) As a result, some players quit without realizing they can fix this issue with very little effort.