I have attempted to mix someone else's recordings before...it was the worst 3 months of my life, and at the end I was ready to sell all my gear and get a job in a garage fixing cars. I know nothing about fixing cars, but felt i would know more than I did mixing records. I just couldn't do anything with at all....everything sounded completely different, everything was recorded so dry and unprocessed that I had no idea how they wanted it to sound (even though I knew the genre very well, hardcore punk, the recorded sound/tone was not hardcore punk!). In the end I got so frustrated that I went out and bought a synth and spent the next 2 months making stupid noisy sounds just to seperate my ears from anything remotely musical. It proved to be very therapeutic, and once I learned how to love life again, I came back to try and mix the recording...almost instantly the depression set in again.
"Why does the snare sound like the drummer is hitting it with crayons?"
"Why do the guitars sound like they were set at 0.5?"
"What the hell is with the vocals...I've seen this band live and his vocals are as brutal as a Rottweiler, why does he sound like a poodle?"
"Why oh why oh why did they record the bass direct from his Rat distortion pedal?"
I eventually gave up...I had no premises to get the band in to re-record anything and they had no more money for a studio, hence approaching me to mix it for free. 3 months later and nothing to show.
They eventually took it back to the studio after finding more funds and it was mixed very quickly. Suddenly they think I'm an idiot. I heard it...it was far worse than any of my own attempts at mixing...and they loved it!! I couldn't believe it...the band had told me at stages of my mixing that they were happy with how it was sounding, but it just wasn't right.
So yes...I'd certainly never recommend having someone else work on your mix, you might end up with an idiot like me
But seriously, I will never try and mix someone else's recordings. Soon after I did a session with the said band as an apology for taking so long...as it was being recorded the sound was coming together and didn't really require mixing. one idiot in the band suggested taking it to the previous to mix it. Great job they would have done, quickly balance the levels and claim the work as their own. Needless to say they were ultimately happier with the recording I did and my faith in myself was slightly restored.
More studios need to take a leaf out of Mr Albini's book...works wonders for me!