My iPhone 4S won't play sound on speakers?

When I play music out loud it works just fine but when I try to play a YouTube video there's no sound?

Well it sounds like if it works for your iPod application and not the YouTube application then it would be a software problem nothing hardware. Restore and update your iPhone (if iPhone software is out to date). The new iPhone software does not include a 1st party app with YouTube in it. So you have to download a 3rd party YouTube application. This is why it may not be working. It could be hardware which i doubt though. When the phone is unlocked try turning the volume up and down using the side volume buttons and if it just says "ringer" volume then your good but if it says ringer then headphones in parenthesis under it, it is a software glitch as well and the hone thinks that headphones are plugged into the device. I had this problem and as oddly as it sounds i put a vacuum cleaner to the headphone jack for a minute or two and it fixed the problem. Hope this helps!

Have the same problem, but it's not that there's no sound. In my case, the youtube videos' sound is being 'transferred' to the 'ear speaker', i guess you can also call it "phone call speaker"? Anyway, I don't know what's the 'cause of this problem, still looking for answers.

If you plug in headphones the sound will work. I you plug in headphones and the sound doesn't work, unplug the headphones and plug them back in and that should work. This works with me. I don't know about other iPhone 4S's but this technique works on me

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