In the screen shot it says "Generic usb audio". Do you have a usb audio device connected to a usb port?
As far as a driver for the IDT internal audio system, as you know Dell does not support XP on the Vostro 1550, so there is not an IDT driver available for your system. Normally Windows would be able to install its own native HD audio driver but XP came out before HD audio did so there is no native HD driver in XP like there is in Vista and 7.
You could try installing random IDT XP drivers. If you get one to work please let us know which one. Be sure to have Service Pack 3 before trying to install HD drivers.
IDT 188.8.131.5241 for Vostro 3350 & 3550 (these have the same audio hardware as your Vostro 1550 so this one might work)
IDT 1.0.6402.0 for Precision laptops
IDT 184.108.40.20662 for Studio 1555
IDT 220.127.116.1147 for Vostro 1320, 1520, 1720:
IDT 18.104.22.16877 for Vostro 3300 3400 3500 3700
IDT 22.214.171.12496 for Vostro V130
Another idea is to let Windows go on the net and search for a driver. Your audio hardware is IDT 92HD87B1.
Hello sir," This is the conversation between Dell executive (supervisor) and the poor customer who purchased Dell Vostro"
Dell Man :what is the problem
Poor Customer: i have purchsed Dell Vostro 1550,Am facing a issue with the speakers,the speakers output is very low
Dell man : sir Dell vostro will not support for multimedia it is not designed in that way for watching movies of listening music it is not designed
Poor customer :what is the solution for the problem
Dell Man :two options sir
1) Buy external speakers
2) Buy another laptop for multimedia purpose
The moral of the story is do not buy Dell vostro model laptops