Why can I not hear vocals on my computer?

I've checked my head phones, they are fine. I can hear vocals on my iphone and ipod with them so it's not that.
I've cleaned the hole you plug the headphones in.
I've checked my settings but nothings coming up.
Any suggestions?
I've been listening to instrumental music for the last few days and I just want to watch videos, and listen to music with lyrics.___. I have to turn the sound way up and I can hear the vocals, but the actual music will be way too loud.
When I watch videos, the voices are really crackly, but I can hear them, I just have to turn my computer on max volume.
I'm on Windows Vista, Sony Vaio

Your computer should have a built in audio level mixer you can use to adjust the volume of each source as required.

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