Thanks for playing and thanks a ton for the feedback!
Hmm the powerups should be able to stack on the ammo bar, like this:
(above, the 'piercing bullet' and 'poison cloud' have stacked on the 4th bullet of the clip)
Are you able to see that when your powerups stack? If not, that's probably a bug, sorry about that! (If you can remember which powerups cause this, that'd be very helpful, no worries if not).
Thanks for the feedback on the music and sound, that's definitely something I can work on in the future. Thanks for playing!