Yes - in short you're correct.
The term 'Summary' only provides an indication of the sentencing limitations of the court to hear the matter - summary in most states means 12 months is the mximum they can dish out to you (handled by a Margitrate's Court) - and who would want a longer gaol term for a loud exhaust!! You get less for armed robbery these days!
So unless they (the infringments) are a 'strict and absolute offence' (which does reverse the onus as you quite rightly said - are many traffic offences; then the Crown carries the burden of proof. This should be explained in each respective state EPA Act. Mondi maybe correct in respect to the state EPA regualtions in his state - not sure which he's from and I'm not going to read all the various state legislation (I get on here to forget about that kind of stuff so I can spend productive tme fantasizing about exotics). I would be suprised though, in respect to a vehicle noise level - that you would carry such a burden - they may be able to issue you a ticket on the spot, but in most cases, they still need to prove their case. That's when you may present your argument.
Note in Qld and Vic the police will be recording your conversation on the side of the road - so don;t admit anything, if you're going to contest the matter. In NSW it's against the Privacy Act for police to do this will an LD Warrant or your informed consent.
Of note is that even some local councils can issue noise infringement notices - depends upon their by-laws and regulations.
I think though in most cases - if you pass the 'attitude test' to swing the officer's discretion in your favour ( I let them lecture me and then applogise before they start writing (!) - but don;t make any liabious admission of guilt, in case you want to contest it later) - and you are not hooning around and annoying people - you would be unlucky to get breached for this. I'd only use the 'loud facility' on the system on track days and away from shopping centres and residentail areas. But like Mondi said, if there is a crack down they may be out to prove how effective teh crack down is - and this means the more charges they can lay the better they look, so be careful and show some good judgement and things should be OK. Good Luck
It puts the lotion on its skin, or it gets the hose again.