Linux-compatible Laptop add-on sound cards?

[Leo Mauler]

For the last few years I've been seeing these add-on sound cards for laptops which promise better sound than the standard laptop sound chipsets.  And on the same lines as my recent thread on printers, I know of at least one person who still uses Windows98 on his ancient laptop (PII) because Linux doesn't have drivers for the sound hardware on his laptop.  I've popped out his hard drive, dropped in another one, and installed Ubuntu 8.04, and while he's impressed, the "no sound" problem is an upgrade-killer.

Does anyone know if there is an add-on sound system for laptops, for which there are Linux drivers?  While a laptop upgrade may sound better, even the used laptops with PIII CPUs are over $400, while the add-on sound cards generally are under $50, making the Linux switch "with sound" much more attractive with just the add-on sound to an existing perfectly functional laptop.

Even the folks who have Linux-supported sound on their laptops would benefit from these cards (if Linux drivers exist for them), which generally have features on them not found on most laptops, such as surround-sound features.