The vibrate feature and the sound out are separate functions - changing one should not affect the other. Make sure your volume is turned up, either with the occasional volume control on screen (a slider, usually while playing music or video) or using the volume buttons on the left edge of the iPhone. They should both control the volume. You’ll know it’s doing something if the Bell icon shows up on-screen, along with the dots/squares to indicate the current volume level.

If that still isn’t getting any sound, then either your headphone jack is stuck thinking there’s something plugged into it, or your built-in speakers are faulty. Either way, it’s worth taking the phone back to the store for inspection and possible replacement.