This plugs in to the headset jack on the iFly gps. I then connect a cord from the Boosteroo output to the aux input on my PM3000 intercom. The Boosteroo has an on/off switch only. It amplifies the sound to about double what it receives from the gps. You control the volume with either the iFly or the intercom. It uses 2 each AA batteries for power. Cost is about $25.
I had tried every volume setting I could find and replaced the audio cord as well. My intercom works fine with my phone, but not with the GPS. Now I can hear the alert "twangs", altitude alerts, and even music I have stored on the 740.
Perhaps since you are flying open cockpit you could use an external speaker if you do not have an intercom.