May I suggest the Bose X-10 as it comes with a remote.
You could actually drive two of them with a splitter.
In a small weight room, it would be "loud".
And as Tom stated, you can buy extra remotes.
Regards, John
I should elaborate, as the reasons behind my comment about the remote control were not clear.
As shown on the Ebay page, the remote control kit has small plastic remote control (transmitter) which is somewhat akin to a remote starter for your car. Based on its size, apparent construction and the price of this kit, my intuition is that this is fragile. It also has a telescoping antenna. Although it obviously has a battery, there is no mention of what model battery is used. So here's what I expect will happen:
It'll get trashed by feet or barbells.
The antenna will eventually break.
The battery will die and it may die often.
Replacing the battery may, or may not, not result in the housing being put back together properly. Screws will get lost in the process.
If worn by an employee, there may be "sweat out" which will render the transmitter useless.
If worn by an employee, it will end up in their locker or at home.
Summary: this is not a good solution for the client.
I don't expect the OP to agree with this (hell, he may not even come back to read it) and state it solely for those who want to at least consider what we all have been cautioning about.