Welcome Battur0710,
Thank you for using Apple Support Communities. Based on your post, your Beats Flex volume is lower than expected. We'll do our best to assist with this.
To update the firmware for your Beats, use the following: Update the firmware of your Beats product
Should it persist, please reply back with the below answers for more help:
- Does this occur when using other devices?
- Does this happen with multiple audio apps on your iPhone?
- Have you checked for Headphone Accommodations? See: Customize headphone audio levels on your iPhone or iPad
Wishing you the best, take care!