Spending all the time and effort into learning how to make your own rom just to get some increased ringer volume seems like the wrong way to go about it. I'm not saying it's as complicated as plotting a mars mission or anything, but it's surely not something you can have completed over a weekend when you're starting from scratch.
With that in mind, here's some other options.
You have the option of taking out the sim card that your work issued you (in the phone) and using it in another phone, although helicopter's are really really loud and also windy, whether or not another phone could really be sufficiently louder is something you have to consider as well. But this is the "simplest" solution. Maybe your old phone was loud enough, and it's also a GSM phone.
If your work and the noise problems are in a stationary location, like I dunno, maybe some kind of warehouse, using some kind of bluetooth accessory might be viable. I don't mean a bluetooth headset, I mean something connected to a speaker or a PA system. You can get use a stereobluetooth headset adapter that will plug into any regular audio device such as a set of computer speakers or an AV jack and when your phone rang, it would play through those much louder speakers. If you went out of range from say walking to another location, what happens automatically is it disconnects and your regular phone speaker is used of course.
I've been thinking about how fun it would be to have speakers on my bicycle since I ride it to work when the weather's nice, and sometimes I'll go out for rides with friends. Hearing some tunes while riding might be fun, and I don't like the idea of wearing headphones while riding a bike.
I've been looking at several options for bluetooth speakers, and I've seen some that are even wearable. This might be an option as well. Of course if you want to take a call and use the phones speaker for their voice, you'd just press that little bluetooth icon when you take a call. If it's too loud to be able to hear the phone ring, I'd have to assume it would also be too loud to be able to have a phone conversation though, so there is that.