Had this Exact issue, do the below and it should work.
1. Go to your rgl_config.ini location
1.1 - C://Users/Name/Documents/Mount&Blade Warband/rgl_config
2. Right click, make sure 'readonly' is unticked
3. Open it up, set that use_winmm_audio to 0
3.1 - use_winmm_audio = 0
3.2 Save the file
Here is the part people keep forgetting to include, the part that will solve your issues
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4. Go into where your game is installed, right click on your exe file and tick RUN AS ADMINISTRATOR
4.1 What I found was that if the changes were not authorised by an administrator the game reset the values back to their original state, if you check run as admin it will use the values as default.
Should work but give me a shout if it doesn't fix it xxx