1) the sound drivers often want extra services to load properly, i had this with my AC97 soundcard, and just manually added each one to modules.conf. Various mixer things etc... Search on the net for the parameters, and you should find what you want...
it's for the alsa drivers, which i'd guess you're not using.... but hey.. it may help
2) log in as root. modprobe shouldn't be on a user path. it'll be ni /sbin usually.