If it's more than a 2-sided board then unless you have a layout of the internal traces, you're probably out of luck. You can try putting it back together and hope that all there is in the broken area is ground and power planes and that they're not shorted -- it depends on how much you love your mixer, and how much money and effort it'd take to replace it.
What GT Electronics said about drilling the end of the crack to stop it from propagating.
I would do the following:
Mechanically fix the board. I would do this by scuffing areas of the board that don't have traces, across the crack, and then epoxying some scrap pieces of PCB material across the crack. This, plus the crack-stop hole will prevent the board from breaking up worse. Use good epoxy; name brand in your area (good hobby/consumer-grade ones in the US are Devcon, Gorilla, or Bob Smith Industries; actual industrial-grade would be better).
Electrically fix the bad traces. My inclination would be to scrape the solder mask off of the traces around the crack and patch with wire with diameter about half the width of the trace. GT Electronics is probably right about using wires that start and end at component holes -- I'd probably still scrape the solder mask, and solder the wire down on top of the trace. It's probably not at all a concern on that board, but in the event that the traces have high frequency or noise-sensitive signals on them, following the original traces will get you as close as possible to the original designers intent.
Just for personal improvement, review what you did to break the board, and try not to do it again! I've learned innumerable lessons from innumerable mistakes, but only where I've done so purposely.