You are not alone ... although the thread has apparently gone quiet, you might try looking at a string at the xps audio forum

http://www.dellcommunity.com/supportforums/board/message?board.id=xps_desk_audio&thread.id=599

 

Clearly neither Dell nor Sigmatel have figured out how to address this problem and the solutions reported by some of the more technically savvy folk, don't work fully (and didn't work for me). 

Brilliant!

 

Followed your suggested link and read through the thread.  Wasn't too confident on the registry alteration so kept that as a last resort, but tried replacing the drivers with XP ones.  Took a few goes with fingers crossed but it seems to have done the trick.  Have to say I find the advice along the line of 'disable the enhancements' slightly patronising - almost an update of the 'turn it off, turn it back on again' schtick.  Makes me thankful that there is a knowledge base of consumers that seem to offer more useful solutions.  Will keep an eye on this topic in case an official fix becomes available, but I thank you for your assistance, and that of the contributors to the original thread :)

Delighted to be of help and perhaps you can reciprocate.  Please enumerate the steps you took and the source of the XP driver that worked for you.  I tried the registry approach (with fingers crossed and prayers) and was unsuccesful in solving the problem. I sure would like to be able to use the line-in and mic on XPS 410/Vista 32 OS.  Thanks in advance,
Gary

Ok, I followed the thread which suggested (on the 2nd page i think) replacing the soundcard drivers.

 

If I remember rightly in the device manager window I just deleted the existing driver, rebooted (and as a result had no sound at all).  Then I unzipped the new downloaded XP driver into the dell drivers folder on the hard drive and ran the only application in that folder.  Another reboot, and a bit of fiddling in the sound options and there it was. 

 

You're looking for an 'input monitor' box which needs to be clicked to unmute it.  As for which driver, I just entered my service tag in the download center, put the OS as XP not Vista and looked for audio drivers.  I was after the sigmatel one.  I did have to install the driver manually as it were, in the driver folder because when I tried the automatic update of drivers the computer decided it wasn't needed.  The only problem I have is the loud popping sound on start up.  This was mentioned by other people in that thread and is a bit like when you first turn on a guitar amp, not like a shotgun going off or anything - but its abit annoying and you might want to check the speaker output is low initially.  My issue was compounded by the fact that I'm not online at the moment (this is my flatmate's PC) so there was a fair amount of memory stick usage.  I'm sorry if this is a bit sketchy as I'm not always the most logical or methodical worker and I hope its a help.  Put it this way - if I can do it anyone can!