Comment 15 for bug 488103

Well even though you definitely have the MBP 5,1 (and the same Subsystem & Vendor IDs - which should automatically match to the mb5 model) it looks to have chosen the mbp3 model instead which would only happen if it was overriden when the module was loaded - can you see what the contents of /sys/module/snd_hda_intel/parameters/model is? Otherwise you could change the last line of alsa-base.conf from:

options snd-hda-intel power_save=10 power_save_controller=N

to

options snd-hda-intel power_save=10 power_save_controller=N model=mb5

and manually specify the model yourself.