This is an older post with many ideas for resolving this issue and here's how I resolved mine. I have a Galaxy Note 3 and had the same issue for nearly 6 months until I finally figured out what was going on. I'm sharing here in case others have a similar set of circumstances.
1. I have a...
As someone else pointed out, the System Volume is likely your problem. In my case on the Galaxy Note 3, it was set to zero as well. To fix:
Go to Settings > Sound > Volume (see the last control labeled "System").