The 230kHz signal is the Class-D switching (a special kind of PWM). It should be filtered out! You don't want that going into your tweeters. You're right that you can't hear it, but you could be hearing sub-harmonics or some kind of side-effect/resonance, especially if you are feeding an amplifier or something other than a speaker. Or, the audible noise could be something else entirely.
If you have a 2-channel 'scope check to see if it's out-of-phase on the other output (it may not be present differentially). And if you don't have a speaker connected, check it with a speaker because the speaker-load is part of the filter. Or, your 'scope may have an option to look at the difference between the 2 channels.
it is also interesting that the waveform does not appear to change much when the amp is connected to an input.
Does the audible noise change? (Is the audible noise coming from the MP3 module?)
It could also be power supply noise. If you have an appropriate battery, try running it of a battery... Or, try running it from your car battery (without the engine running so there's power-supply noise).
I believe the problem is to do with my bridged amplifier output. the two terminals are both at 2.5v so there is no potential between them. this works fine for a speaker output and effectively doubles the potential power from +5v to -5v.
Right. Except, doubling the voltage gives you 4 times the power (+6dB) because when you double the voltage you also double the current (Ohm's Law). (Power is Voltage x Current.)
Is there any way i could convert my bridged amplifier output into a normal one? i know that one can do something with capacitative coupling...
**Right.**A capacitor will do it. DC current can't flow through a capacitor... You're making a high-pass filter... To get the full frequency range,
With a speaker-load, you'll need around 1000uF. (Of course, the positive end goes to the amp-output.) Lower-value capacitors will block the bass (you can look-up RC filters if you want to calculate a value).
Depending on the amplifier design and the source of the noise, you may get more noise with a single-ended output. i.e. If the noise, such as power supply noise, is identical in both output it get's canceled differentially (with a bridged connection).