Thanks for posting in the Apple Support Communities huyenfromnijmegen,
Checking your Sound Check setting was definitely a good step to begin with.
You might also be able to make adjustments to the Equalizer to achieve the volume or sound you're hoping for.
Change the way music sounds on iPhone with EQ, volume limit settings, and Sound Check.
* Choose an equalization (EQ) setting: Go to Settings
> Music > EQ.
* Normalize the volume level of your audio: Go to Settings > Music, then turn on Sound Check.
Change the way music sounds on iPhone
If that doesn't help, Apple is always interested in hearing about features, options, and changes you'd like to see in the future. You can share feedback, here: Apple - Product Feedback.
Cheers!