I've a C-Media sound card - CM8738 and 5.1 speakers.
Everything seems to work pretty good, but there is a problem - two speakers are mixed - subwoofer is a central speaker, and central speaker is a subwoofer for Ubuntu.
1- Run in terminal
2- You are looking for the code that looks like this-
3- It may not be exactly the same, but we want to change only the channel-map order
Notice the order on lfe and front-center, lfe is the subwoofer, and front-center is your center speaker, if it displays another configuration, just swap position of
Let me know how this works for your version.