grab the collection plate with your left hand. raise your cupped right hand up to and over the plate, then lower your right hand to the plate. move your right hand fingers back and forth a bit jiggling the coins. it will sound like you dropped a whole handful of coins.
if no coins then just flutter the bills a bit, it will look similar.
pass the plate with your left hand.
As a person who is not a believer (but who has sat thru many a church service), I always toss in a dollar. (When I was a kid and I’d stay with family members who were church goers (Catholic) my mom would always give me a dollar to put in. This was in the 70’s, so worth more. When in middle school/high school when I’d have sleepovers with people who I’d end up going to church with in the morning, I got the same dollar and I gave it in the same sense. “I’m supposed to put something and I don’t want to be rude.”) I always felt I got more side-eye from the not going up for communion thing.
And by the way, yay for great church sound systems. At least two Unitarian churches in the Houston area use that sound system for singer/songwriter music on Saturdays, and it is awesome!
Any of the churches I’ve been a member would pay you to attend. Oh, wait, they are already doing that…
I understand your sensitivity. I feel the same way too. If you really want to blend in, take some singing lessons, and belt out the hymns. For my money, good singing forgives any other lack of contribution, and to top it off, if you sing like you know them everbody is going to assume that you are a full-committment supporter.
Or maybe that’s just me.
I’ve been in exactly the same position, attending church services for audio checks. If I ever had a question about the service or any customs, I’ll always ask the priest or minister beforehand.
As others say, there is no need to be embarrassed. Smile nicely, and pass the plate along. Everyone else will think you are a great Christan because you are obviously pleased with how everyone is contributing to the Lord’s work. No one needs to know that you are secretly happy that they are getting funds to pay you.
The vast majority of churches where I have been a member, and most where I have visited, have made a point of saying something like “If you are visiting with us today we do not expect you to participate in giving, you are our guest”. Giving is a tenet of the belief system - if they were doing baptisms that day would you feel pressured to join? Same concept.
I’m another one of the belief that you have no obligation to put anything in. If you did work for a non-religious charity, would you feel compelled to donate part of your salary back to them? You are a paid employee (contractor) of the church & are there working; you are not there for spirituality. It may not be your denomination or even religion & you should not feel obligated to donate to them.
I know that places like amusement parks & stadiums have different rules for employees. They are allowed to bring in food/drink & or buy it in the back at discounted prices. The point being that rules are not the same for employees as they are for attendees; should not be any different in this case.
I say give $100.
Of course, it’s an expense, so you need to add $100 to the invoice. Then add your profit and G&A markups. 
Now THAT would get tongues wagging.
*Heavens *No! 
Since you’re not supposed to give in order to win the approval of those around you (thus says Jesus), neither should you give in order to avoid their disapproval.
Get up there before the priest or reverend, tap the microphone, blow into it, say “two, two, two” then hand the pulpit over. Wear black, including sunglasses.
I’d say if you did a particularly good job you should TAKE money from the plate.
You deserve gratuity for a job well done.
“Forgive us our trespasses as we forgive those who trespass against us” is an excellent test for sibilance…sibilance.
You’re stealing my material 
No, I don’t think you should feel obligated to make a donation. You’re not there as a participant in the service, you’re there as a contractor doing the job you were hired for. It does occur to me how much of the service to you have to attend to determine how well the sound system is working (or how badly it’s not working)? Wouldn’t just hearing a couple of prayers from the podium and some songs by the choir, organ, musicians, etc. in a full house be enough to let you hear all you need to hear? If so, why not just take an aisle seat and when you’ve heard enough, leave?
Oh, and try to get work at synagogues. Since Jews consider handling money work and that the Sabbath is a day of rest, they never pass a collection plate. But if it’s a conservative or orthodox congregation, they would expect you to wear a yarmulke in the building. But they’ll have a wooden box of them on a stand near the doors for people who don’t bring one to wear. And they won’t mind if you forget to put it back in the box when you leave.