I know exactly what you are talking about!
It's hard for me to find the alto and even to hear the alto on some records. I sing soprano in church (I do it well too), but my voice sounds better (if singing alone) in my mezzo-soprano range.
What I have done is, instead of singing the soprano when listening to records (which would be easy) I focus in on the alto, find the part, and sing along.
You might not have all the movements right, but your ear will catch on and you will say "Huh that doesn't sound right...rewind." Rewind that tape and figure out where the alto part moved.
It helps alot! Now when I'm out (especially since like all my friends sing) I can, with some work, add the alto part. But, in my listening to songs I have gotten much better at singing tenor. I told my friend this who is an amazing singer and on New years we were at this concert and she was like WOW you really can sing tenor now. Hahaha.
Also if you can find the tenor or soprano note you can find the alto note. Move up or down a step.
Usually, I just end up singin octave of soprano alot of the times. People don't know what you are singing around you, but it still matches the music and sounds good. So if you want to sing out, but don't want to strain in your soprano range just sing octave of soprano.