I have been cleaning my molds in my ultrasonic cleaner for years. My Ultra Sonic cleaner is an older H.F. Industrial model and has a 2 Liter SS tank and outer case. For aluminum molds I have used distilled water and dawn, mineral spirits and acetone. Be extremely careful with the flammable solvents! If I am using flammable solvents, I do it outside on a concrete surface or my steel welding table placed away from anything that could be burned! Having covered the safety aspect, I have never had a fire (yet). I also have a 16 gauge steel plate that covers the top of my cleaner.
I completely disassemble my molds as far as possible. All of the solvents do a good job of removing oil and grease making casting easier. For old well used molds the burned on stains don't always come off, but the molds cast well after cleaning. The down side is that sometimes molds need to be broke-in again after cleaning.