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Is the phone plugged into the bike? On my bike I have a 12 volt power socket where I can charge my device while riding, do you have anything like this?
Is your bike excessively loud?
I can think of no reason why execration would affect your device's volume. The only two things I can think of that might have an effect like this would be if the device was plugged in to a charger and something with the higher RPMs would affect the voltage output and some how that was having an affect. Or the vibration of the bike is causing it again somehow. I have a tank bag that is see through so I can have the phone laying on the tank in a way that allows me to use navigation so my phone is in direct contact with the bike and vibrating along with it.
My only suggestions would be to try without charging, if you are. Try with the device in your pocket and not in direct contact with the bike, if it is.
I brought up the excessively loud point because our ears become accustom to sound levels. So what is loud at low speeds is barely audible at higher speeds when engine noise increases. This tempering, if you will, lasts for extended periods of time. I experience this on my drive home. With the window cracked the ambient sound created on the highway from the wind whipping past my window and traffic causes me to increase the volume on my radio. Even when I arrive in town on the other side the volume remains high but I hardly notice the difference until I go to the car the next day and find it blaringly loud from the previous day. This would be easy to verify by knowing the volume level when you begin your ride and looking at it again at the end of the ride. You device my not show a percentage but you should be able to do something like take a screenshot of before and after and compare the two.
It would actually be counter intuitive for the volume to decrease with an increase in ambient sound. Where as I can see no reason that speed and/or acceleration would affect volume.
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