The Starkey Handbook on “compression” has a good explanation of the effect of attack and relaxation constants of compression on hearing aid function.
The attack and relaxation of compression is a very different phenomenon than hearing aids changing their amplification volume or changing their degree of droning noise suppression or their directionality when transitioning between quiet and noisy environments with or without speech present in an environment. Music program settings are special because they need to use longer attack and relaxation constants than programs designed for listening to speech.
Since I’m a bit rusty on hearing aid knowledge, fooling around with other stuff lately, I’ll leave detailed explanations to other folks that are truly knowledgeable on the topics I’ve touched on. The post referenced below has a link to the Starkey Compression Handbook in it. The Handbook does a good job of explaining how hearing aids work, at least as far as compression goes.
See also the following post for the attack/relaxation settings that ReSound recommends its HCP’s use in setting ReSound aids.