How can I exclude Sound Recorder files from Music library
I have found a solution to the first problem thanks to the reference by beeshyams. 
MortPlayer Music allows you to choose which folders to draw your music from. Hence, in my case, I removed all the .nomedia files from Sound Recorder and only allowed music from my Music folder in MortMusic, thus leaving the SoundRecorder functionality as it was (i.e. recordings show up in the Recorder App) but not in MortMusic, where only my desired .mp3's show.