Heavier gases tend to slow down the speed of sound. It makes sense if you remember what sound waves are: pressure waves. It's easier and faster to create a high pressure front with lighter molecules than heavier ones.
The reason why monoatomic gases are faster is because they don't have bonds that will act as springs that absorb some of the compression. Again, it's easier and faster to create a high pressure front with balls than 2 balls attached together with a spring.
Not sure if this stuff will be tested on the MCAT since the speed of sound in an ideal gas will only vary with temp.
For the MCAT, I'd say knowing how speed of sound varies with temperature and phases is good enough.
