Merging audio from multiple videos into one audio file

Hello,
I'm holding a virtual carol concert for charity on Friday 11th December. I majorly overestimated my technical abilities though, hence reaching out for last minute help.
I have about 20 friends filming themselves singing their parts (soprano, alto, tenor, bass) to 5 carols which I've given them a piano part for (this will be playing through headphones so only their voices will be picked up on video). They are sending over their videos by the 30th November.
I appreciate this would give a very short window of time for this last minute project if I hoped to have it completed by the 8th December. But would you have time to work on something like this please? And if you did, how much would you estimate it would cost? I'm not sure how many hours of work it would be for you.
In an effort to reduce costs, I can probably work on editing the videos together myself with the one finished audio track. But if you would be able to provide two estimates - one for doing the audio work, & one for if you did the audio & video, I would really appreciate it.
Thank you so much for your consideration.
Best wishes
Katy