I have several MP3 and FLAC files in my playlist. Everytime a MP3 song finishes playing and the next song in playlist is FLAC the playback hangs. I must stop playback using the "Stop" multimedia button on my keyboard and start the playback anew to get the song playing. This behavior is 100% reproducible. I use GStreamer as the Phonon backend. Also I use the PulseAudio sound server. With the VLC Phonon backend this issue doesn't happen.
You should report issues with a backend to the phonon-backend-* tracker, and it is a duplicate. Please see https://bugs.kde.org/show_bug.cgi?id=302652#c22 and provide the necessary information in that bug *** This bug has been marked as a duplicate of bug 302652 ***