It is the defect of most browser-based games: weak music accompaniment. And it is not a fault of game creators: one can be a great coder, graphics artist or level designer, but too few of us can compose a non-boring music, suitable to our activity.
Recently I've downloaded HUGE (about 50 GB) archive of so-called "tracker music" (or module music) from modland.com. Inserted several hundreds modules into VLC's playlist and ran it in background.
During very heated mission, one theme started to play, and if was very appropriate to gameplay.
It may sound cakophonic a bit, but as background music is very "stimulating" during harsh combat.
Some other themes in "Farmer" folder also seems to be appropriate.
As most of "demoscene" music, these modules are royalty-free.
To play these, one may need some player capable to play modules - they aren't like MP3. My suggestion is VLC from VideoLAN, but some others like foobar2000, mikmod, winamp etc may suite.