I´m trying to translate this:

Istersen bir ara sesini duysun.

Does it mean: Sometime if you wish, let him hear your voice.

In fact, you translated it correctly.But "ara" means "to call/phone".

If you want, call him/her,(and) let him/her hear you voice. 

But also it can be translated as yours.

That´s where I got confused. Because of the ´bir´. But I found some where ´bir ara´ is sometime. But either way, it means the same.

I cannot explain what "bir" means there. Maybe, we use it to emphasize the verb but I am not sure. 

And not same meaning

"bir" is in adverb position and means something like "(at least) for once"

call him/her for once so s/he can hear your voice.