You can burn MP3's as regular audio, and it will occupy the limit of 74/80min on a cd, and play in all cd players.
However, if you burn them as MP3's in the data format, they will go in since they have WAY better compression and you can fit hundreds on there because you have 700mb to work with instead of 80mins. This method though, will only work on compatible cd players [i.e. a in-dash car cd player that supports MP3's as such]
DVD's hold 4.7gb before you even talk about dual layers, so if you just want to transfer songs from your computer to hers, burn them as MP3's [data] you can fit probably a couple thousand on there but just make sure she has a DVD-ROM to read the DVD!
Hope that helps.