Depends on what sound you need - If all you want is "A" sound, there are pezio buzzers would fit on a normal base.http://www.adafruit.com/products/160
They'd run off a 3v coin battery though which generally are not cheap. Thin ones though like the 2025 just barely fit underneath the base itself. A lot of work, but it would serve a few purposes making the base both magnetic and weighted.
I considered these for my noise marines way back when but never followed through. They'd probably make an appropriately annoying buzzing sound though.
Anything with an input would take considerably more space. The record able greeting card guts mentioned above are available on ebay for about $2.00 per. They'd probably fit in just about any vehicle (ok probably not a venom).http://www.ebay.com/itm/like/400770550491?lpid=82&chn=ps
Those 2 metal cylinders on the right are pill type batteries, and replaceable for whatever that is worth.
I've been integrating single LEDs into some of my character's bases and it works pretty well. With those however I cheat by basing with acrylic crystals from the dollar store and drilling a hole up through the base and crystal to integrate the LED. You can't really do that with any speaker technology.