I looked into this some months ago, and found the following
Headphones to non 3.5 jacked units
As described, a 3.5 female stereo plug can be spliced into the output wires to the exsisting speakers
Pros
You can within reason, use any aftermarket bud/ monitor you wish - but be aware the huge quad driver unit that looks good in the picture, atually sticks out of yourr ear by 50mm
Cons
Modern spacesaving headphone cables now ony use one wire to carry the the 3 required signals
L , R & Ground - This is achieved by resin / enamel coating the individual wires before wrapping .
This makes resoldering a pain, as you need to remve this coating before the wires will resolder
as the wires are so small, they destroy themselves easier than removing coating!
If you do get a 3.5 jack soldered in, you then need to locate it somewhere dry and away from water ingress / tucking it into your neck / back of the helmet, isnt the most comfiest place lol
3.5mm jacked units
Thankfuly a lot easier, just put your buds in .. job done
Audio quality 1
Unless the buds, monitors come with expanding foam tips, you will get noise bleed from the exterior of the helmet with the std rubber tips, this can be wind and engine noise.
on non expanding foam tipped buds, they are rendered usless over 30 mph
Audio quality 2
So youve splurged couple of hundred quid on some all singing, dancing, bass enhancing 20Hz - 20000Khz killer buds.. matched the speaker impedance ( because you read up on it
)
plumbed them in, and they sound awesome.
But the battery life of your unit has halved ..
Fact of life i'm afraid.
The 40 hrs battery time quoted by the manufacturer, is derived from playing in an anchoic chamber at half volume playing some middle of the road monotone tyrolean folk song.
Driving a boggo standard single driver bud
Give it Rammstein at volume 11 throug a 3 driver bud, and it will be 20 hrs or less. Those bass drivers need power, lots of it lol! Coupled with the mid and soaring treble drivers, Nav &
phone dutys mean somthings got to give ....
yup, your headset battery,
You could look at the flat speakers in the helmet.. and there are some gains to be made here
Position
Are the speakers inline with your ears ...
sounds daft, but helmets along with most mass produced stuff, are set up to cover the 90th percentile
Which means they will be ok for 90% of wearers, if your not in the 90% then they aint going to work properly.
so the speaker centres, may be nowhere near your ears, so a simple adjustment may be all thats needed.
That said, if there miles off, theres nothing to stop ypu carefuly, and at your own risk, removeing some of the polystyrene, and relocating the speakers.
Distance to your ears
Now i cant explain the maths behind this and if you can, then go for it
But again, the speakers are mounted in a recess in the helmet, which is some way away from your ears,
This is to give the ears some space to sit, and ensure there not compressed by the helmet / speakers
However they could be a country mile away. by moving the speakers closer to your ears, will give noticable gains in loudness
without having to increase the volume . You can often just add an additional pad behind the original speaker that came in the original kit
As a last resort, there are better drive units out there ,
The supplied ones are built to a price / profit point for the oem, not you.
So cutting edge and efficient, they prolly wont be.
But by changing them to some efficient and better quality units. coupled with the positioning tips should see bigger gains
NB caveat 2 wil apply (soldering )