I am trying to play some mp4 files that were captured on my samsung solstice phone. The videos play fine in gnome mplayer, but there's no sound. I am able to play .mov and .avi files with gnome mplayer with no problems.

are you 100% sure that the mp4 files have sound ?
and what codec package did you install for mplayer ?
was it the current "all-20100303.tar.bz2"
http://www.mplayerhq.hu/MPlayer/releases/codecs/

The files do have sound as I am able to play them with sound through pitivi video editor.

I'm not sure what codec package I have installed for mplayer. How would I get this information?

try installing vlc it plays all video formats without any issues and installs necessary codecs too...
plugins required may be known as good,bad and ugly..
But first try to install vlc..

I've tried vlc. When I open the video file I get this message: VLC does not support audio or video with "samr". Unfortunately, there's no way to fix this.

Please advise.

Apparently, VLC considers the samr codec to be encumbered.

This thread at another forum suggests trying mplayer. If it works in mplayer, you may be able transcode it with mencoder, which comes from the same folks.

try installing smplayer
it loads more codes for ff like some new 264's
type smplayer in a terminal it tells you how to install it
enjoy your mp4's with sound

Thanks frankbell and no2498.

I will try installing mplayer when I get home tonight and let you know if it works.

all smplayer is a qt based front end ( a GUI ) for mplayer
and will not install any new or different media codec 's

I do not use Ubuntu so ...
check apt get to see what codec media packages are installed
but most people just grab the all 2010 tarbal and put them in the codec folder
/usr/lib/codecs or /usr/lib64/codecs
-- or some still use the old folder --
/usr/lib/win32

Same issue from a Linux Mint 17.3 Rosa fresh install.

I could hear sound from a basic sound test in the Preferences --> Sound --> Output, and I could hear sound in my chat programs, but just not with any video file, regardless of whether I used VLC or Banshee. I also noticed my Mic wasn't working even though it was enabled and it looked like it was producing sound in my chat program. So I went to Preferences --> Sound --> Input and tested my Mic there. I didn't see the bar go up until I maxed out the volume. Now my mic for my headset works in the chat program, and this caused my sound to work on video files. Isn't that weird? I tried uninstall/reinstall VLC and VLC Pulse packages, and that didn't help.

SOLUTION: So the fix was raising my Mic volume in the Preferences --> Sound --> Input settings. Funny.