We include as few as possible kernel modules in the rootfs to keep the image size small. So really any added kernel function needs an extension. Extensions are typically packed by function, i.e. alsa, graphics, touchscreen...... Normally when looking for sound you would load alsa, when you load alsa.tcz, it would automatically grab the modules as a dependency.
tce-load -wi alsa.tcz
tce is an extension browser where you can search, it should become obvious if you do a search for alsa in the repo.