There is a device called a Motochello Speaker Bridge that converts the bike's speaker output into a signal that can be fed into speakers, amplifiers or other audio systems. You can read their own explanation as to why you'd need a converter. The device costs $99 and I do not seek to dissuade anyone from buying their easy to install system. If you don't want to spend that much and fancy more of a DIY project this mod might be for you. I do not accept any responsibility whatsoever for any damage you may cause to your motorcycle.
I already had all the bits I needed that I stripped off my 2005 R1200RT when I sold it. You can do the same for much less than a Motochello.
The benefits of this mod:
a) you can choose your own bluetooth transmitter to send audio to your helmet using up-to-date bluetooth standards and codecs (or feed directly into earbuds)
b) you can control the volume using the wizz wheel
c) you can use the bike's S-VOL: feature to automatically increase volume with speed
d) you can quickly and easily switch off the speakers without having to go through the menu system (or use speakers at the same time as bluetooth or earbuds)
What you'll need:
1) a 'speaker level to line level converter' that converts the voltage and impedance of the Alpine unit's speaker outputs so they can be used safely with earbuds or a bluetooth transmitter. Converters like this one and this one should do the job and cost less than a tenner.
2) a switch to place near the front of the bike to easily switch off the speakers. You'll need a double poll single throw (DPST) type to switch both left and right circuits together. I chose THIS round waterproof one from CPC for £1.50.
3) a bluetooth transmitter (not a receiver) that will send the audio that is normally output to the speakers to the helmet. Choose one that needs an external power supply so that it will switch off with the bike. This will replace the built-in bluetooth unit so it can be left turned off. I chose THIS ONE from Amazon.
4) a soldering iron or suitable connectors to wire into the speaker wires; suitable insulation or heat shrink tubing; cable to reach wherever you place the speaker switch.
5) suitable connectors to go from the line level converter to the bluetooth transmitter (the transmitter may even come with them)
Here is diagram to help:
