Good morning everyone,
I think that the 1M pot might be a good idea just in case, or to have several different resistors to test what the gain should be. The mp3 output will be at fixed level(max volume), so there isn't room for adjustment there.
I will also need to hear the music at the same time from the mp3 source. I was planning to use just a simple 2€ headphone splitter, one end going to the amplifier and the other to the VU meter circuit. If this is somehow a bad idea, I could use suggestions for proper solution. Could there be signal attenuation?