connect airport express to router wifi not apple

The wireless connection of AirPort Extreme and Express to wireless routers seems to be an Apple proprietary format that is not compatible with other brands. Occassionally a user gets their Express to connect wirelessly to another brand of router but that does not happen very often.

now I need to connect my hi-fi via an airport express (via wifi of course) I canconnect "airport express" to a wifi router does not Apple via wifi?

Yes, the AirPort Express can "Join" virtually any wireless network if you know the name of the network, the exact type of wirelss security that is being used on the network and the wireless network password.

The Express will work for AirTunes and/or allow you to connect a printer when it is configured this way.

But....the Express cannot provide any additional wireless coverage for the network when it is configured to "Join" the network, so it will not "repeat" or "extend" the wireless network in this type of setup.