No sound on my iphone 6 plus

1.

If you have a computer transfer to videos to the computer & do the videos have sound?

See -> Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC

Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support

2.

iPhone 6 Plus is an outdated iPhone (9 september 2014), that may be the cause of the problem.

If the problem persists, visit an Apple Store or an Apple Authorized Reseller in your region for assistance to diagnose the problem. And they could offer a possible solution.

You will first have to make an appointment due to the pandemic.

See: Contact Apple for support and service

Contact Apple for support and service - Apple Support

tsakani193 wrote: "The maximum capacity is 100%"

That implies the battery isn't the original. Was it replaced by a qualified technician using a genuine iPhone battery?...

About genuine iPhone batteries

Repair Provider Verification - Official Apple Support

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tsakani193 wrote: "I believe so though, I didn’t get my iPhone straight from iStore... I bought it as a preowned iPhone"

Apple's iPhone User Guide says: "If your iPhone isn’t working as expected, you can try restarting it by turning it off, then turning it back on.":

Turn iPhone on or off - Apple Support

...But if no luck with that, your choice is to either have a qualified technician look at it or buy a newer model iPhone that will run the latest iOS 15:

iOS 15 and iPadOS 15 are compatible with these devices

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Thanks for your feedback & you're welcome.

tsakani ~ Does your iPhone 6 Plus still have its original 7½ year-old battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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