You need to make sure that both Pure Music and Amarra are not running at the same time. And you may need to re-configure Amarra with the right audio device as sometimes these programs get confused if they can't get exclusive access to the sound device.
Also, you may want to try rebooting your computer. For troubleshooting purposes, you'll want to make sure neither application launches on startup.
Also, you may want to try rebooting your computer. For troubleshooting purposes, you'll want to make sure neither application launches on startup.