Rhythmbox doesn't add/import MP3s from ~/Music on Ubuntu Gnome 15.04

Based on our discussion, I assume that the database was corrupted.

Delete it:

rm ~/.local/share/rhythmbox/rhythmdb.xml

or rename it:

mv ~/.local/share/rhythmbox/rhythmdb.xml ~/.local/share/rhythmbox/rhythmdb.xml.bak

And start the import again.

In order to be able to decode and play MP3 files, You need to install required decoder which is not installed on Ubuntu by default, And are provided in the package called "ubuntu-restricted-extras".

Run this in a terminal to install that package :
sudo apt-get install ubuntu-restricted-extras

After installing those, Try to import your MP3 files again.
If failed (Which is your problem now !) it's probably because the database was corrupted.

So as A.B. said, You can just delete the database and re-import :
rm ~/.local/share/rhythmbox/rhythmdb.xml

At this stage the shouldn't be any problem with Rhythmbox importing mp3 files.