The AMR files can't be played on Poweramp. You'd better convert the amr file to mp3 thus to play the audio files on Poweramp. To convert amr to mp3, you can use the online music converter like Zamzar, CloudConvert, Convertio, OnlineConverter, etc.
For the DRM-free iTunes Music, why not directly connect you Galaxy S7 with your computer then find the downloaded music files and transfer it. But for the DRM-protected music, you can only convert these music files to MP3 that playable on Poweramp app via some DRM removal