Is there any simple application (or built-in in Mac OS) that allows me to record sound on a Mac? I'm referring not only to voice input through the mic, but also to sounds issued by the system.
A simpler solution would be SoundFlower which acts like an output to which system audio can be sent, then used as an input source in any audio recording program, even quicktime. If you want to be able to hear sound from your speakers/headphones, you'll need to run the sound flower bed app, which takes sound flower's audio coming in from your system, and forwards it to your speakers/headphones while still being able to use it.
This answer is similar to the one XAleXOwnZX gave, it just has a link for Audacity and a tutorial.
If you want to do it for free you can download Audacity and Soundflower (if you have a mac) and record the sound straight off your sound card.