After training, the mind is relaxed and completely stress free, that is if you trained hard enough. Everything is better in this stated. Life is better, music is better. I trained really hard this past year and my resting heart rate went from 80 or so beats per minute to high 50s BPM.
In a normal mentally relaxed state the heart rate is also at a relaxed state. This is different after training. If you have exerted yourself properly during training, afterwards, your mind is extremely relaxed...but. Your heart rate is relatively high. Drugs and alcohol can have the same effect of relaxing the mind yet speeding the heart rate...and I bet there are many stoned and tipsy music lovers aplenty.
Also the body is still warmed up, unlike first thing in the morning, when muscles and ligaments are cold. Training creates a load on the body, this load generates heat. Blood absorbs the heat and distributes it away for cooling. This warmed blood will inevitably end up on your eardrums and create more elasticity in the tissue. If flight and fight response of human evolution is to be believed, then in this state, your hearing is likely to be more acute. The latter is my opinion. The former is a fact.