There are two reasons why speaker cables are not shielded/screened:
The signal is so powerful that any interference would not be noticed.
Speakers are not very sensitive; it takes a lot of power to create sound on a speaker
This is why speakers are connected to an amplifier.
Input to amplifiers are very sensitive and so input should use shielded/screened cables.
P.S. If you have a 100W amp the good ones use about 48V. So you have at least 2A, with peaks higher. Peaks might hit 20A but for a very short time. To hear this your speaker cable needs to be thick.
Distortion
Distortion can be caused (in this amp, assuming everyting is connected like it should be) by one of four causes:
- Speakers gets overloaded - small speakers cannot handle a lot of power so it produces distortion if you try to play the music too loud, it probably would distort the low frequencies first as low frequencies require bigger cone travel.
- The amplifier output gets overloaded - if the chip (or the power supply) cannot produce enough current, then it will clip the waveform and produce distortion.
- Not enough voltage - 8 Ohm speaker needs 8VRMS to produce 8W of power. As you use a single supply and capacitor coupling, it means that the peak output of your amplifier is about half of the power supply voltage. So, to get 8VRMS (11.3Vpeak) the power supply as to be at least 22.6V, in practice it probably would need to be more than that because the output transistors in the chip drop some voltage. Otherwise there will be not enough voltage and the waveform will be clipped (the chip cannot have 14V on its output if the power supply is 12V)
- The amplifier input get overloaded - the input voltage is too high for the amplifier chip This is not very likely for your project, but I included it for completeness sake.
Buzzing
The input picks up the electromagnetic field from the mains wires inside your house and amplifies them as if they were useful signal. To reduce the hum, you should put the amplifier in a metal case to shield it. It would still pick up some hum when the input is not connected to anything (or worse, connected to a wire which is not connected to anything), but that is not really important.
Also, the hum could be coming from the power supply (if you use a non-switching power supply) if there is insufficient filtering (not enough capacitors).
Resistor in series
It will not work - putting resistor in series with the input you will just lower the volume while putting the resistor in series with the speaker will reduce the output power because you will be wasting some power on the resistor. The amp will just start clipping at lower volume.
My suggestion
Find out the power of your speaker. Connect it to another (more powerful) amplifier and turn the volume up until you hear distortion. Note if the volume is flouder than with your amp or not. If it is the same, then you need a more powerful speaker.
If the chip can survive it (read the datasheet), use a higher voltage power supply, especially if you want to use an 8 ohm speaker. Or find a 4 ohm speaker - it would need less voltage for the same power (if your chip can drive 4 ohm speakers).
Best Answer
TL;DR You can split to any number of amplifiers you care to, but only a limited number of headphones. Try it on your particular player and see how it copes, as there will be differences.
A music player output tends to be able to drive a pair of 32 ohm headphones to reasonable levels.
This means that the output impedance of the player is low compared to 32 ohms, often a factor of 10 lower. The output impedance is important as it governs how much voltage drop there is on the output when it's loaded.
The input impedance of an amplifier is usually in the 10k to 100k region. There is essentially no voltage drop, regardless of the number of amplifiers you split the signal to (well, up to hundreds of them).
With a factor of 10 difference in impedance, there could be a 10% drop in output level when headphones are connected, compared to when they're not connected.
Connect a second pair of headphones, and there's a further drop. Compared to the variation in volume of music, and the variation possible due to the volume control, you would still be hard pushed to notice the difference of adding a second pair.
If a small reduction in volume was the only issue, then you may think there would be little problem in splitting the signal to more headphones. However, there are several issues.
Music players tend to have a limited current output. As more headphones in parallel draw more current, that will eventually overload the amplifier, causing clipping distortion. This will occur at a lower sound level, the more sets of headphones are driven.
Music players might go unstable into lower impedance loads, and you would get poor sound quality regardless of volume.
If it's smart enough, a music player might shut down into a too low impedance load, and then you'd hear nothing.