OK, I've had a quick try of that link on the work iMac (as mentioned, running OS X El Capitan) using Safari and JWplayer was behaving on that, the audio was playing fine for me? From my experience of JWplayer (we use it in some of our stuff at work) I'd expect it to behave the same on Chrome and Firefox regardless of platform though there are occasional oddities.
I suggest you go back to the person reporting it as not working on their Mac and find out what version of OS X they're running and what browser they're using.