I installed pianobar as well and ran into the same issue.
I believe that the error you received is related to ffmpeg. (libavcodec.so being a part of ffmpeg)
Unfortunately ffmpeg is blocked from installation, and if I have learned 1 thing about arch, it is that you do not ignore warnings
The message I received when trying to install ffmpeg via pacman was
ffmpeg is in IgnorePkg/IgnoreGroup. Install anyway? [Y/n]
i chose no
A solution would be greatly appreciated