Dont forget to update your Raspberry periodicaly....

It's easy and can increase performances and functionalities !

$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get install rpi-update
$ sudo apt-get clean
$ sudo rpi-update
$ sudo reboot

You could also remove old kernel modules:
$ sudo ls /lib/modules

4.1.19+ 4.1.19-v7+ 4.9.35+  4.9.35-v7+

$ sudo rm -rf /lib/modules.bak
$ sudo rm -rf /lib/modules/4.1.19+
$ sudo rm -rf /lib/modules/4.1.19-v7+


To remove automaticaly old kernel modules :
$ sudo PRUNE_MODULES=1 rpi-update


More informations about kernel releases history:

https://github.com/raspberrypi/firmware/commits/master

And search a modification concerning a specific part as ALSA:
https://github.com/search/advanced?q=alsa