Singer Jensen:

Down the shore everything's alright by jeyhawk
http://jeyhawk-fic.livejournal.com/65228.html

In It for Love by dragonspell
http://dragonspell.livejournal.com/135247.html (writer!Jensen, but contains one scene when Jensen sings for Jared ina reality show)

I would also rec Beatiful disaster by thenyxie, though Jensen is the guitarist of the band and Jared the new singer, Jensen also sings in it and it's an awesome awesome story. It's worth reading it.

http://thenyxie.livejournal.com/548429.html

This one is an AU but Jensen is lead singer in a band and along comes Jared. It has great supporting characters with Steve, Chris, etc. Each next part in the verse is linked at the bottom of the post. This is a classic as far as I'm concerned. :)

Rock Star Verse (http://tempestquill.livejournal.com/17197.html)

I almost don't want to rec this fic because I'm kinda spoiling the ending but it's so worth the read: Lost and Found (http://mediaville.livejournal.com/57649.html).

You should definitely read this one: Watching Over You: Epilogue (http://community.livejournal.com/with_you_baby/5938.html)

and the first part: Watching Over You (http://community.livejournal.com/with_you_baby/5683.html)

Though that part doesn't have Jensen signing it's easier to read them both

# The Song On Track No.5(The Reconstruction Of Jensen)1/2:Jay needs to hear Jen sing
http://mind-share-inc.livejournal.com/1154.html

Break Loose Ranch (http://smokeringhalos.livejournal.com/10404.html) is a fantastic story with an awesome singing scene. Very hot AND romantic.

:-)

mournthewicked has one:

Sing Me Sweet
http://users.livejournal.com/_mournthewicked/279381.html
(sorry, I don't know how to link the title)