If you have an "audio" signal out of the Arduino, a line-level "RCA" input should work. The 0-5V signal is actually quite "hot" and might over-drive (distort) a line-input. It wouldn't hurt to put a capacitor in series with the output. That will block the DC component, giving you an AC signal that goes (approximately) -2.5V to +2.5V. It's possible that the DC is fouling-up the monitor's internal circuitry. A pot/volume control wouldn't hurt either.
Is that a piezo buzzer or a piezo transducer? A buzzer produces it's own sound when power is applied. A transducer is like a speaker, and you have to send it an audio signal.
Have you tried another source to make sure the monitor is producing sound? Are you sure that RCA jack is an input?
While I'm on that subject, I'd love to have more than one channel of sound (possibly just 4 channels of audio mixed together) how possible is that?
From different sources? The Arduino plus 3 other sources? For that, you'll need a mixer. You can build a passive mixer with resistors (or pots) but you'll get signal loss as you add more inputs. You can buy a small mixer, or you can build one. A "summing amplifier" is easy to build with an op-amp. (Audio mixers are built with summing amplifiers.)
Thank you for the information. I didn't know what the voltage range for a TV set is.
As for a piezo, its just a normal one. I have tried plugging external speakers to it and the results were the same as the TV set - no audio.
So you say, a solution would be to hook up capacitors in series? what not a resistor to limit the voltage? or a DAC?
And as for multiple channels, I actually don't want to turn this device into a mixer. Think of a gameboy or old Nintendo where it had about 8 channels worth of sound. I'd like to turn 4 pins (4 channels) on the board, with just 4 resistors (non-pot) if that works.
And in regards to all of these resistors and caps that have been mentioned, what kind of uF and ohmage would you recommend?