I recently upgraded from Ubuntu 14.04 to Ubuntu 15.04. The upgrade went fine, except that now audio plays from the headphones and the speaker at the same time.
My speakers are plugged into the motherboard's green audio jack while the headphone jack is integrated into the front of my computer case. In the sound settings, there are 3 channels: Digital Output, Headphones, and Line Out. However, changing the audio on either Headphones or Line Out affects both the speaker volume and the headphone volume.
On Ubuntu 14.04, the channels had been Headphones, Digital Output, and Analog Output. Changing Analog Output did not affect Headphones and vice versa. Additionally, when I plugged in headphones, it automatically muted the speakers.
I am running on AMD hardware. Gigabyte GA 970a-DS3P, AMD FX-6300, Sapphire Radeon R7 240. The audio is built into the motherboard.
One difference is that I installed Ubuntu 15.04 in UEFI mode, whereas I had installed Ubuntu 14.04 in BIOS compatibility mode.
Additionally, because of a quirk in the motherboard, I turned off IOMMU in the BIOS settings. However, I did this in both 15.04 and 14.04, so I don't think this is the problem.