Apple allows you to re-download any inApp purchase at no charge as long as you are signed into the same account that you purchased the plugin with. You can do this even if you have switched to a new device.

If you are not certain what account you are logged in with on your phone:

1) Go to Apple's "Settings" app
2) Go to "Store"
4) If the account listed is NOT the account you purchased the inApp item with, sign out of that account and sign into the one that you purchased under.
5) Now return to Pocket Tunes

To re-download the inApp purchase:

1) Go to the web tab in Pocket Tunes
2) Tap the home button in the bottom right corner of the web screen
3) Tap the link for "High Fidelity aacPlus v2 Decoder" (If you don't see this link, then you have already purchased the aacPlus v2 decoder. If it doesn't seem to be working, try restarting your iPhone.)
4) Tap "Buy High Fidelity aacPlus v2 Decoder"
5) Tap the button to Buy. As long as you are logged into the account you purchased with, you will not be charged again.
6) A message will appear saying you already purchased the item and to tap OK to download it again. Tap OK

That will reinstall the inApp purchase to your device. See this Knowledge Base article on how to determine that the plugin is installed and working.

Contact support if you have any problems.