Q: sound problems after installing iOS 9.3.5?

Hello, try to reset iPhone by pressing and holding Sleep/Wake and Home buttons.

- Try cleaning out/blowing out the headphone jack. Try inserting/removing the plug a dozen times or so. There is a little switch in the headphone jack that disconnects the speaker when the headphone jack is inserted.

Fix for iphone Earphone Bug

Try the following to rule out a software problem

- Reset the iOS device. Nothing will be lost

Reset iPod touch: Hold down the On/Off button and the Home button at the same time for at

least ten seconds, until the Apple logo appears.

- Reset all settings Go to Settings > General > Reset and tap Reset All Settings.

All your preferences and settings are reset. Information (such as contacts and calendars) and media (such as songs and videos) aren’t affected.

- Restore from backup

- Restore to factory settings/new iOS device.

- Make an appointment at the Genius Bar of an Apple store. Seems you have a bad headphone jack.

Apple Retail Store - Genius Bar

https://www.apple.com/retail/geniusbar/

If not under warranty Apple will exchange your iPod for a refurbished one for this price. They do not fix yours.

Service Answer Center - iPod

A third-party place like the following will replace the jack for less. Google for more.

iPhone Repair, Service & Parts: iPod Touch, iPad, MacBook Pro Screens

Replace the jack yourself

iPod Touch Repair – iFixit

is the "jack" still the same using Bluetooth headphones? Because the sound problem is the same

Wait wait wait... I reset my settings and I had sound, then I went through my settings starting by flicking up on my iPad and clicking the mute notifications button, I went back and no sound, I believe that 9.3.5 made the mute notifications button into a mute all button, so try turning it off.