I started wearing ear plugs after just 2 uses with my Yamaha, albeit older model, as I noticed a significant impact on my hearing despite being quieter than a lot of machines out there. After 2-3hrs each time it certainly added up. It's been a week and I'm still noticing a slight loss. I'm not sure why I went without it as I usually put on hearing protection when using a hammer or impact driver as even short bursts of noise like that bothers me.
I wonder if cutting open the muffler to pack it with something or install more baffles would make a significant difference without sucking anything into the engine.
As for the Decibel apps, they are pretty accurate. I had one on my iPhone several years ago and used it along side an actual high end meter and the results were the same. The issue is the self calibration options in them so you could make it more sensitive or change the range slightly but with nothing to compare it to you won't know if it's accurate or not. As said, it depends on the phone being used as well, dirt in the mic, if there's a case over it, etc. Still, it will get you in the ballpark of the actual noise level and at no cost to you except make you aware you should be using hearing protection.