Probably because all DOS / legacy support in Windows XP is non-existent. XP is now a true 32-bit OS based on the NT kernal... basically what that means is no DOS period. You have a 'command line' interface but that isn't really DOS. What are you attempting to run anyway that still requires DOS support for a sound card?
There is basic support there.. but only WINDOWS support.. no DOS. That means if your games work in Windows XP and use DirectX, you'll get sound. What they mean by basic support is that none of the 'advanced' features are supported. (IE ATi Radeon had basic DirectX support with WinXP default driver, but no OpenGL support. To get full driver, you had to download it from ATi). If the games actually run, that's fine, but there are NO DOS drivers in Windows XP... so unless you have a legacy sound card, you won't get any sound in DOS-based games.