I believe the sound chip in my (1 year, 9 day old) Dell Inspiron N7110 may be giving out on me; I hear intermittent buzzing when the speakers are playing
audio, or when wearing headphones, or having external speakers hooked up. I even hear the buzzing through the TV speakers when hooked with HDMI. I
believe if it is possible to use a usb sound card with internal laptop speakers, it is much cheaper then buying a new motherboard for the sake of a sound chip.
Probably not. The only way to get the signal from a usb audio device into the computer is to run a line from the device's output into the computer's line-in jack. But then you are back to using the laptop's integrated audio.
There was a Soundblaster non-usb model that could internally route the signal back to the laptop -- the old Sound Blaster® Audigy® 2 ZS PCMCIA card. That was a slot card. You did not need a cable, but it still just routed back to the integrated audio, not a substitute for it. Maybe some other slot cards could too but I don't know of such.
The problem is that there is no way to directly connect the usb device to the internal speakers except by cracking open the case and connecting wires to the speakers, and even then the usb device's headphone amp probably could not drive the speakers anyway or properly drive the "sub-woofer". The 3 speakers in the N7110 can use up to 5 watts of power.