The sound card is Yamaha Corporation YMF-754.
This is my workaround which works for me:
Go to sidux's online manual, click on the left menu link:
Graphic Cards, Monitors & miscellaneous Drivers > Hardware needing non-free drivers
Scroll down and find this line:
Yamaha YMF724/740/744/754 audio devices:
fetch the firmware from http://git.kernel.org/?p=linux/kernel/git/dwmw2/linux-firmware-from-kernel.git
and place it under /lib/firmware/yamaha/.
I do not know what is "fetch" means, so I just click the above link and then click "snapshot" to download the firmware (the file is compressed in tar.gz).
Extract the file to get these three files:
ds1_ctrl.fw
ds1_dsp.fw
ds1e_ctrl.fw
Create a directory for the firmware:
# mkdir /lib/firmware/yamaha/
Copy those three files to that directory.
Restart the computer, and the sound card now works.
Problem solved.
