How can I clean my receiver to be able to hear better?

Hi there, smaja84!
Thanks for reaching out to the Apple Support Communities! That is a great question you have about cleaning your receiver and I’m happy to help! Check out this article with information about Cleaning your iPhone. Specifically:

iPhone 6s, iPhone 6s Plus, iPhone 6, iPhone 6 Plus, iPhone 5s

Clean your iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean your iPhone, unplug all cables and turn it off. Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, or abrasives to clean your iPhone. The front glass surfaces have an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.

To clean the Home button (allowing Touch ID to work optimally), wipe it with a soft lint-free cloth.

Also, if you continue to have issues with the sound and have followed the steps in the article If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker, check out this link to Contact Support. This will get you in touch with Apple and they will be happy to proceed with further steps.
Cheers!