One could 'easily' (...FCVO. SMOC!) have a shell-script that accepted a selection of files, calculated[1] the lengths and pop up a hint-list. Then you could select a likely bunch, 'right click' and send their names paramaterised to the associated script for display. Maybe a dumb listing as a guide, possibly repackaged into a clickable list (htmlified dynamically and sent to browser? I suppose a Tcl/Tk solution might be the better (thinner) solution though...). Or develop this in a directly PCMANFM-friendly pluginabilityness to plug the very obvious gap, but I can help you very little in that right now.
I might give this a go at the weekend, though it'll probably not be flexible enough for everyday use by the time I've finished polishing it to my own satisfaction.
Not a 'solution', but I often listen to (or watch) media at 150% speed, so hour-long things only take 40 minutes - and still perfectly understandable for it (200% can be a bit too much, unless I'm rebinging on the way to the 'good bits' for something I know well enough). Apart from making the normal-speed bit actually seem a liiitttlle biiit sssllooooww (in my head) when dropping back to that, it's a handy trick available if your media player allows it. I've got l some radio from this morning
currently piping through my headphones as I write this, and it's hardly making it dilfiluct to tyep thungs att al!!!ñ
[1] Or retrieved from file, having once been calculated. And if you have that, you could also do a reverse-search by popping up anything of "30m+-5m", possibly with additional theme/genre filtering if you have time to set that up as well, to give you randomised suggestions that suit. Back in the day, I had a nice little script called by the .xinitrc to check for console location (or ask me to enter it, for the IP, if not known) via a simple flatfile, with a pop-up console that let me check what other users of note were also logged in on in the same lab's machines ("abc123 @ ServerWell/Next to door") or any othe lab I decided to check (e.g. from the VT100s) to see if there were free seats. It wouldn't take much to convert that idea to your purpose, though in a much less kludgy (and no longer so X11/bashy) manner.