You can use a free open source program called Audacity to convert, edit and modify sound files. It supports Windows, Mac OS and Linux. It can lower the volume if you like. You can modify or replace the files you don't like. Remember that each OOTP update will overwrite any sound files you change, so make a sounds backup folder so you can copy your custom files back to the sounds folder. The OOTP developers can't make the game perfect for every person. I'm running a 19th century league so I don't want rock music. I do agree that a playlist checklist would be a good option. The game will still play wav files made for previous versions.
http://www.audacityteam.org/download/