I just want to chime in hear (no pun intended), as well.
The 5720 is a great laptop but the onboard sound IMO is junk. Not only do I have the headphone hiss like so many, but the richness of the onboard sounds leaves something to be desired. I use Bose noise cancellation headphones and a nice Klipsch Pro-media 5.1 set for external sound and gaming and MP3 playback is underwhelming in both cases. I've enjoyed great sound from the Klipsch and Bose in the past but I'm getting literally half the bass and clarity I've experienced in the past. I opted for a USB sound card as well and the difference is night and day. I went with the Phillips Aurilium. 39 bucks online and the sound blows me back in my seat
The clarity and bass is tremendous in headphones and my 5.1 setup. Headphone jack is clean as a whistle as well with zero hiss.
And yes I've played with the eq and sound settings in Realtek sound mgr til I'm blue in the face.
Get a USB or PCMCIA sound solution if you buy this laptop. It is a must.
Oh and one more complaint. If I remap my ports in the Realtek Manager, they map back to default after every restart so I'm constantaly remapping the headphone jack to get rid of the hiss. Another annoyance with the on board sound.