Music volume equilisation...?

[Combusted]

I've put a number of songs onto a CD, as a compilation Cd for work...
But some of them are so quiet, and cannot be heard, especially when the inflatable is in the pool (Lifeguard here).
And some are too loud...

Is there any way of making them all the same volume as one already on there? since one seems to be perfect..

Thanks in advance.