/diy/ - Do It Yourself

hello /ohm/, I have a cursory understanding of electronics and I am a decent solderer (have done loads of repairs on my dad's racing motorcycles after crashes).

I am currently taking care of a mentally handicapped individual, and I'd like to lower the max volume their device can go to.
The volume control is digital, so not a knob you can just glue shut, so I said I might know a solution.

Attached is a photo on the inside, it is my understanding that if I solder resistors in line with the cable and the connector for the speaker, it will lower the volume.

Am I correct in that assumption? How do I determine what kind of resistor I need, trial and error?
Can I use a multimeter to measure the speaker's resistance and make an educated guess that if I put something around 20% of that in ohms infront of it, it'll reduce the volume by 20%?
Is it safe to buy resistors from aliexpress?
Would heating up of the resistor not be an issue?

Reduced battery life is not something I'm worried about, but if there's a more elegant solution I'd love ot hear it.