Why would on-board speakers sound scratchy but headphones sound OK?

Why would speakers sound scratchy, but headphones and external cabled speakers sound OK? I have a Dell Studio 15 (1555) running windows 7 Professional.

236GB used, 214GB free space
Intel P8700 2.53 GHz Processor
64 bit OS
4GB memory

The built-in speakers used to sound good but now they are too scratchy to understand. However the headset sound is good. According to the Dell Support Assist utility, the sound card is "ATI high Definition Audio Device" and "IDT High Definition Audio CODEC."
Microphone test -- earphone playback is OK, on-board speaker terrible.

You Dell Studio 1555 laptop uses IDT HD audio driver for the built in laptop speakers.
The ATI HD audio driver is only used if an HDMI connection to an HDMI monitor (with an HDMI audio/video cable) is used.

possibly a driver conflict with IDT audio, built-in speakers may be malfunctioning or dirty (gotta have them repaired if it's a hardware problem and not a software problem). just listing the possible causes for the scratchy sound.

So far:

1) The scratchy built-in laptop speakers use the IDT HD audio driver.
2) The good HDMI connection (I used to a projector) used the ATI HD audio driver.

Q) Which driver would the good-sounding headset sounds be using?

According to Device Manager, both my IDT and ATI drivers are up to date when I search automatically for updated driver software.