iPhone ringer problem

I hope I'm allowed to post this sort of thing, but if not then please remove it.

I've been having a problem with my iPhone for two weeks now, and I've heard of this happening to others before but didn't think I'd come across it. The volume bar for the ringer disappears, and I can't hear videos, music or incoming or outgoing text messages. I can hear my ringtone go off with my alarm or incoming phone calls, and I can hear when I make calls or receive them. My phone is stuck on headphone mode, and I can't get it out of it. I tried everything I could from tutorials and fix-it sites online, but haven't gotten a fix for my phone.

Something did happen when I went to 'Settings > Sounds' and I changed and browsed through my ringtones, and I could momentarily hear incoming texts and songs. But then it would go back to doing that. My last resort is to open up my phone and check to see where the issue might be coming from, and whether or not it's the volume buttons that are the issue. I'd love to go and send my phone to get fixed but I need it with me for work. Has anyone had a problem with this too or have found a way to fix it without opening up the phone? I have an iPhone 4 with the latest iOS, the phone is a year old and it's never been damaged in any way. If I can't fix it, I'm going to have to upgrade my phone to a 5 or 5s, and I want that to be my last option.