Hi @Anonymous and Welcome to the Forum
Some ringtones are lower in audible volume -> Have you tried an alternative tone or is it too quiet on all the tones.
Has this always been the case from day 1 or has this just begun.
Check the phones speakers for any lint.
Is the phone in a case which may be hindering the speakers ,or is their any form of protective shield affixed to the screen which also me be hindering it.
Sometimes turning the phone off and then back on after a few seconds can clear glitches.
Their are several Internet sites with tips / tricks and I believe some apps that may help.
If these don't help then perhaps approach the Manufacturer via
http://www.samsung.com/uk/support/ or an O2 Shop can send it off to their Repair Centre but we do generally recommend the Manufacturers route first as they know their software and hardware better.
Let us know how you get on.