Just wondering if anyone actually knows the notes that they are singing when they come up with a melody for the vocals in their songs, or if they just sing whatever sounds good?

I always assumed I would need to know what notes should be sung with what rhythm parts - to find harmonies and whatnot, but so far I just sit around and hum different things and try to improv some lines while playing until I find something that seems alright and then go from there.

If you don't figure the notes used for the vocals out before hand, do you go back and sort them out?

It seems like for writing cool solos/riffs knowing this would be good because you could have your guitar doing the melody at some points instead of the vocals. I remember learning how to play the vocal melody for "hey jude" and its like, that would be kind of cool if I could do that with any song.
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i guess it can be applied, in all my tab books, the vocal melodies seem to be in perfect time, not the most common note durations but enough to fit the meter properly
if you can get by well enough without taking the time to figure out the notes and the harmonies and their relationship to the other parts, it doesn't matter really. having a good base in theory and knowing how notes relate to one another can make writing melodies and harmonies much easier, and can allow you to do more complex things sometimes, but in music, it all comes down to your ears and whether you like what you hear or not.
Generally you sing what sounds good, then find out the notes, this is the same with harmonies.

That's not to say you couldn't figure out what scale and notes to sing prior to writing, but it works good both ways.

However, it is imperative that you know the scale the melodies come from when it comes time to auto-tune (if you're into that kind of thing)
This water's dark and coldGod's not where you hopedThis moment come and goneIt's time we all moved on
I generally go with whatever sounds good. When I hit obstacles or challenges, I revert to my safe haven of extensive theory knowledge to work it out... especially with harmonies.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.