Open Audacity
File > Import > Audio
One the track dropdown, "Split Stereo Track"
Copy the band channel, Paste
Now you will have 3 channels
Mark the two band channels as L and R
Mark the vocal channel mono, and use the tracks slider to mix to taste.
Save
Export
(actually, you could probably get by just marking both channels as mono and get the same result)
If you want to get fancy, you can make some subtle eq changes on the L/R chanels to create a fake stereo feel...But I'd think the mono version will sound pretty good.