Are there are free or in general really good audio software for making original songs or remixes?

Garageband is awesome for composing original songs. Stay away from the loops though.

I’ve heard good things about Audacity
although i’ve never used it.

@patg7590 Audacity is ok but it’s not really a music-maker per se.. as much as it is a sound editor imho

Fruity Loops, Reason, Virtual DJ.

I use Fruity Loops and Reason on a daily basis.
You can sample songs, make your own beats.. And they come with cool things like Vocoders and all different types of mixers and preloaded samples.
Also, if you have instruments, like a midi-keyboard, you can record right into the program and play with it. Pretty cool stuff.

LMMS is good for mixing but I’m not sure if that’s what you’re looking for. Jokosher might be more adapted to your wishes, but that’s not available for Windows yet which I’m guessing you’re using.

Wikipedia has a nice list too.