I sing along, using the following formula:
* If it's a standard that everyone sings along to, do it and do it loud. Grace, Too ain't the same without the crowd belting it out along with him.
* If it's a song with a big crowd-pleaser lyric (like Bobcaygeon or, unless I miss my guess, Melt), belt out that lyric and keep it down otherwise.
(Corollary: on Bobcaygeon, do NOT yell out "that night in Torontooo" every ten seconds. If anyone nearby does this, administer smack to back of head.)
* If it's a lesser-known-but-kick-ass-song, sing at something like normal volume, or slightly under.
* If it's a lesser-known-but-quiet song, keep it the heck down. Mouth the words.
* If I forget the lyrics, shut up.
* If _Gord_ forgets the lyrics, help him.