Bluetooth headphones hissing and distorting sound

Have you tried the new headsets on different devices, it could be that the other end of the Bluetooth circuit is causing the issues - the computer Dongle or Phone

The main reason that i asked the question is that it is unlikely for five devices such as headphones to have the same issue, it more likely to have the same issue due to either an incompatible or faulty common item, the Bluetooth dongle in this case in most likely, but it could just be a local electrical/electronic interference source, (TV, Microwave oven, cordless telephone etc).

It could also be that the Bluetooth output is not passed through the equaliser / Sound Processor of your source device where as the speakers that you are used to hearing are.

There is of course a chance that you have a particularly bad batch of the headphones, or even that they are not very good, (did you hear the quality of the units before purchasing them?.

I would suggest you can try to connect, either elsewhere and / or to a different source to actually place the fault's Cause

So the headphones are two piece devices, a "Bluetooth transceiver" and a standard set of headphones that you plug into the transceiver - in this case what I said above is still valid as the transceiver is the possible part causing the issue or the bluetooth dongle or the connection between them - the good news is that the headphones themselves are fine as you have proved

One of my Bluetooth headsets has the bluetooth components built in and has a separate audio cable option, if yours is like this the Transceiver is inside the headphones rather than a separate item