Is there a reason that alsa-driver does not depend on linux-sources. I have been building an unstable tree from scratch as a test. It broke on alsa-driver not being able to find a kernel (as I didn't install one).
add the following to your /var/cache/edb/virtuals: virtual/linux-sources sys-kernel/<your-kernel-sources> e.g. sys-kernel/gentoo-sources
i guess this was already fixed, as it is in the DEPEND...