How to reduce an audio signal to proper input level?
Old tape recorders (does anyone remember them?) when put into "record" would amplify the sound of almost inaudible stuff until it got a decent signal, then it went into "normal signal amplify mode". It used what's known as AGC - automatic gain control and maybe this would work for you.... but beware - if you are expecting hi-fidelity don't raise your hopes too high. An AGC is a compressor (much used in modern music like trance and dance genres) and it will alter the dynamics of normal music and worst still it'll shift the dynamics of a lot of classical stuff to "pop" levels.

Basically, it doesn't matter whether you are talking digital or analogue - two different music levels (500mVRMS versus 1250mVRMS) will cause the same issue - you want to turn the output level up on the 500mV signals -  it's an 8dB loudness change and that, to the ear, is nearly 2:1 on loudness (1 bel or 10dB is a doubling or halving in loudness at 1kHz)

Either live with it or introduce a circuit that can compensate for the 8dB loss but be aware that you will lose signal dynamics on the louder music.