Tricky question. The computer determines the maximum quality of sound the speakers will determine the available quality.
Lets say you have a PC with standard sound card hooked up to the best speaker system in the world. The PC wouldn't be able to enhance the sound to any appreciable degree. On the other hand if you have crappy speakers on a great sound card, it will still sound like crap.
For the most part, if your not a audiophile, the sound card in most newer computers will work fine. It will be up to the speakers to reproduce the sound. Most TV's have better speakers then monitors do on account that the have the space to include bigger sound drivers. In fact most monitors don't have speakers at all.