You have the 1/3 rule, you should not drill a beam for first 1/3 of it's span, so only the centre 1/3 can be used, also when running low voltage cables (230 volt) all other cables must be also rated at the maximum voltage which is supplied, so although the speakers and TV coax is classed as extra low voltage, to be run with low voltage they would need to be rated at 230 volt which is simply not going to happen, with metal tray it is permitted to fit a divider and extra low voltage one side and low voltage the other, however since the tray is earthed and surrounds the cables, the two separate voltages are in a Faraday cage so there will be no capacitive or inductive link between them.
You do need to use some common sense, two cables running together for 2 inches is not going to allow much of a capacitive or inductive link so both low and extra low voltage cables running through the hole in a TV stand is not really a problem, but if there is a series of holes then there will likely be a problem.
With AC supplies the feed should be what is called balanced or matched, at 50 Hz this is unlikely to cause a problem in most cases, however with LED lighting the traditional method of wiring a two way switch has been found to allow enough energy transfer with inductive and capacitive linking to cause the LED to glow dim, as a result it is common to include a bleed resistor in the bulb to remove this small current and stop them glowing.
With the very small currents involved with speakers and coax you can expect to have problems, using Ali-tube power cable will likely reduce the problem, I have never tried using it, the idea of the cable is it can be used in walls without RCD protection as drilling or nailing through it will cause a short circuit and trip a standard MCB, it has not really caught on, but if it is impossible to separate mains and data, then the ali-tube cable will likely stop any 50 Hz RF from being transmitted from the cable. Personally feel that's using a sledge hammer to crack a nut, but it is an option.
