Hi, what is not clear is where you are plugging the headphones, in the
speaker unit or in the machine, some clarification is required because
if you are plugging the phones into the machine then the machines have a
problem.

I assume you are plugging them into the speaker unit so the first thing
I'd try is the leads, try waggling the ends of the leads to see if
theres a broken or loose wire.

I would guess the speakers are 'self powered', meaning that they
require a power source to get them to work, be it battery or wall wart
adapter. If this is the case then they will contain a audio power
amplifier to drive the speakers, to me it sounds like one of these
amplifiers is at fault, in which case I would be tempted to try another
pair of speakers.

You can check them however, power the speakers up and try plug the
signal lead (not the power lead) into the earphone socket of a mini
Walkman, alternatively you can use the line or aux SIGNAL out on a CD
player or tape cassette.

*Do not be tempted* to use the speakers output terminal of an stereo
amplifier, the signal level is far too high and the impedance far too
low for this purpose.

What we are doing is using a *low level audio signal* to feed into the
speakers so we can try them, this will point us in the right direction,
obviously if the speakers still crackles then they are responsible.

Davy