Are you sure you executed the following command:
apt-cache --names-only search linux-backports-modules-alsa | cut -f 1 -d ' ' | xargs sudo apt-get remove --purge
since you may have multiple versions of backports-modules-alsa installed.
You probably also need to make sure there are no alsa 'updates' modules - what is the output from the command:
find /lib/modules/`uname -r`/updates
Are you sure you followed all the instructions in comment #5?
Are you sure you executed the following command:
apt-cache --names-only search linux-backports -modules- alsa | cut -f 1 -d ' ' | xargs sudo apt-get remove --purge
since you may have multiple versions of backports- modules- alsa installed.
You probably also need to make sure there are no alsa 'updates' modules - what is the output from the command:
find /lib/modules/`uname -r`/updates
Are you sure you followed all the instructions in comment #5?