You could run it through a compressor or mastering suite when you're done. Or just do a good job of matching the levels. :)

Ok so from your answer that tells me I cannot depend on the auto gain. There is a compression software I can purchase but does this mean I will need to record 2 times to get the final mp3's volume normalized?

www.digitaldjtips.com

I use autogain and find it works pretty well.
But some things will always need to be adjusted further.

That's a great resource, thank you. I was looking into if the volume could be done inside of Serato but like with Audacity, years ago I was using the Goldwave software to do the same thing, it's the same procedure so I will go that route again, appreciate the help guys.