Alright... so this isn't going to be the best way, but it's the only way I know how to do it. Use Audacity. It's not going to be perfect, but it'll help bring out the vocals more. How well it works depends on the song type.
Find an instrumental of that song and slap it into Audacity. Then take the original song and do likewise. Beatmatch them. After that you have three methods, each doing different things:
1) follow this guys instructions(You don't have an instrumental):
3) follow this guys instructions(You do have an instrumental):
2) follow my instructions which are different, and may produce different results(You do have an instrumental):
Take the instrumentals and the original song and beatmatch as before. It is important that they are as closely matched as possible. Highlight the instrumental version, go into the Effects menu, and select "Invert." Mute the instrumental track. Take the original song and copy it again into a new track. Mute the original song, not the copy. In the Effects menu, select "Remove vocals" from the effects list. In the dialog box, click "Okay." Then select "Noise Removal" from the Effects menu. In the dialog box,and inside of that, click "Get noise profile." Do not click "Okay." Exit out of the dialog box.
Now, mute the copied track. Un-mute the original song and in the Effects menu, select "Noise Removal." In the dialog box, click "Okay." Un-mute the instrumental track. tinker around with the audio levels to get the best result.
As aforementioned, this is not a perfect way to isolate vocals, but it definitely helps *slightly. It may work better it you set the songs to mono before to do all that work, but I haven't tried that as of yet.
If you feel like trying alternate methods, you can check out the Wiki: http://manual.audacityteam.org/o/man/tutorial_vocal_removal_and_isolation.html
Best,
Krenton