There is another suggestion in the threads here to use foobar2000 to lower the volume, but it didn't do it enough for me. I used Audacity. Backup the files somewhere then open all the originals in audacity then for each open file do the following:
Press Ctrl-Shift-E (Export File shortcut)
Make sure file type is still OGG etc. Hit OK.
Hit YES to replace the existing file.
For edit metadata either edit or add the following:
replaygain_track_gain -8.00 dB
That was enough of a volume reduction for me, you might wish for more. One caveat.. before trying this I had run the files through the automatic tools in foobar2000 which I guess had added the two replaingain_xxxx fields you see in the screenshot I've attached. I think just the one tag should suffice though I guess you could do exactly what I did and run it through foobar2000 then tweak it down further in audacity.