Nah, no custom theme. It's chicken and egg but I wanna' say it's a "hidden bug" that only comes to light with the addition of the position slider, and that it has been latent since day 1. Tough to tell, I can put the initial App volume to max, then when I move the slider it gets louder still. With the volume on low, moving the slider jumps the volume very loud. Moving the volume bar either direction makes the volume go very quiet, far more than a micro nudge of a volume bar should, so some obscure effect is triggering.
I gotta say, it's tough. You're Swirely Awesome or something, but the end result is a miss for me unless we can do what I call "solve the crazy problem with a crazy answer." (I see that in my job all the time, the "problem that makes no sense has a solution that makes no sense". However if we stumbled on a solution, I'd leave it your Swirelocity to describe it, maybe. Sometimes life has no explanation, only results!!)
No other audio app has ever done that on me, so I dunno what magic combo of perfect forces is at work here, but I'll try to poke around and find you something you can hack to bits to get a fix.
Edit: how are you "playing the mp3 at all?" Doesn't that require an engine of some kind? (Tell me XP doesn't have native hidden support for mp3's!). So is it like the slider, some fluke of a Snacky efficient shortcut you are using? I have an idea - lemme go dig up Trout, if Trout behaves then it's not (wholly) my box, we'd start corralling the problem.