Sound keeps playing, although deleted in IDE (List of Sounds)

*Catroid version:* 0.7.0beta-1305-debug
*Phone hardware:* Galaxy Nexus
*Android version:* 4.2.1

**What steps will reproduce the issue?**
* open any Catrobat program in Catroid
* go to list of sounds
* add a sound (e.g. a longer playing mp3 file)
* tap the playbutton - sound starts playing
* delete the sound with the actionbar item

**What is the expected output?**
playback is stopped, and sound is deleted from list

**What do you see instead?**
sound is deleted from list, but playback is resumed as long as the activity is visible (will stop when clicking back- or home-button, switching to costumes via spinner, tapping the add-button and so on).

this is only an issue when deleting the sound with the delete icon in the actionbar; with a longpress on the sound, the playback is stopped before the delete-dialog appears