Do they work ?

Yes! When you turn one of them on, they make sound - if connected to power, at least.
I have not used one enough to declare whether they help turn on the bite. If they "work" I wouldn't expect it to attract fish, but to possibly get fish present more in a mood to eat - like you smelling some good BBQ about to come off the smoker.
I have two now. Should I sell one?

Yes! Especially during the spawn. Releases Barry White music underwater to really get the bass going! https://youtu.be/x0I6mhZ5wMw

Would today's black bass be more into some good rap?

Most days in tx with it turned on I could see fish all day come up within a few feet of it to check it out..... Does it get you more bites, who knows, but it gets more fish around your boat.

That said at home in Iowa I havent really noticed the same, Im convinced that besides the bass being larger in tx there are just many many many more of them on each lake.

I can tell you that when you have it on many times you will see fish swim right up to it only to dart away when they see the boat or you .They do work for sure to some degree .

It cured my cancer.............

I've had both kinds of days where it made a difference and then not. There is no doubt it attracts fish - whether it triggers the bite is another thing. I have had many times when the cormorants would swim up & peck the speaker. I would say it's biggest benefit is that it helps mask the Area 51 electronics pinging in every direction plus it may mask the trolling motor to some extent.

Here's what I've seen with mine...

1) Do they work? Under specific conditions that work. You don't want to run the unit in 3 foot of water when sight fishing. You want to use it in deeper water when you can see bait fish schools and a few fish running through them to feed. Turn it on and watch how they respond. I've seen it create totally anarchy and I've seen it work like tear gas.

2) Think of it as a tool: Right place, right time, right job

3) When to use it? Experiment, but I do this...when fishing a tournament, I go to my spot fish it hard for my bites without it on. Then I turn it on make the same pass with it on fish the spot hard for a few more bites. Then I turn it off make another pass and see if I get bit anymore. If not, I move on.

It works if you use as a tool and not keep it cranked up and blaring fish noises for no reason.

Thats how I use it if its on its at least half way if Im in 3 feet or 25 feet I dont care, I definitely dont think about it, change it or mess with it. Just turn it on and leave it all day. Maybe it will mask my tm or 360 or live scope or radio.

It's better than wearing a mask .

it has caught more fishermen than fish.

Don't you just love marketing?

I have one. It cost me a giant yesterday on Richland Chambers. While digging out the net, the line got snagged in my hydro wave speaker. I tried to unwrap it, but the leader broke before I could get the line unwedged from between the trolling motor and the speaker.