Are you using the alsa drivers that are (1) in the kernel, (2) part of the alsa-drivers package or (3) compiled and installed from sources?
If it's 1, then you may have mis-configured the kernel options.
If it's 2, does the package version match your kernel version. The driver package installs the modules under the kernel version the package was built for. For example, alsa-driver-1.0.9b_2.6.13-i486-1 puts modules under /lib/modules/2.6.13. You probably need to include the alsa code from the kernel or compile from sources since this isn't a path your new kernel can use.
If it's 3, you probably need to re-install the modules so they're available to your new kernel. You may need to re-run configure and make prior to running `make install-modules`.
WARNING: I have had problems with sound while upgrading to 2.6.15 also and I'm not pretending to have all (or even most) of the answers