Volume changes in time to the music with HD-25 headphones.
When I am listening at low volumes on my Sennheiser HD-25 headphones connected directly to the phone's 3.5mm jack the volume goes up and down in time to the beat of the music. It's quite subtle but I find it unlistenable. It doesn't happen with other headphones connected directly to the 3.5mm jack, with the same headphones connected via a USB-C to 3.5mm dongle or with the same headphones at high volumes.
I don't remember this always happening - has anything changed with the audio in a recent software update?