Frequency range s of digital noise s

Pretty much whole spectrum. Very high frequency can get in and produce audible frequencies by mixing on any non-linear element or by "rectification" in components, such like op-amps, that have different slew rate for positive and negative transitions.

Pss, personal computer clocks might be in gigahertz range but computers also contain switching power supplies that operate in hundreds of kilohertz range and buses operating in hundreds of megahertz. In addition when signal is not continuous, but changes pattern (like data on computer bus) additional frequencies, called sidebands, are produced (since it is modulation). All these frequencies interact with each other creating even more frequencies - ending up in one big noise that Al, picked on his radio. I'm not sure what is Sound Application, but as I said before high frequency digital noise can affect audio band many different ways.

Any cable can pick-up electrical noise including speaker cable. Shielding is based on skin effect and does not work for lower frequencies. The key is to keep cables as short as possible, since they become effective receiving antenna at about 1/10 of the wavelength. Twisting wires help at lower frequencies but becomes ineffective for higher frequencies when pitch of twist is comparable to wavelength. Keeping computer away, plugged possibly into different supply phase, using short cables and good shielding is the key. Power supply conditioners can help since overall system noise increases jitter - hence analog noise.