Can I only use my iphone4s as an alarm clock if leave it switched on!?

Hi, firstly not sure what you mean by 'but doesn't seem to work as an ordinary phone would!'. If you're switching the iPhone off completely i.e. holding down start/stop on top and then sliding bar this will turn off the phone completely and no alarms (or notifications) of any sort will work. Alternatively, if you're just pressing the start/stop briefly to clear the screen then your alarm setting should work. Oddly though, I've found that the count down timer will only work if on screen - something for future investigation!!

Since you have an iPhone 4S (with Siri) you could just ask Siri (hold down home button) something like "set alarm for 2pm today", or anytime of your choosing; Siri should confirm and all should work fine.

One other, probably obvious, point is to check your volume is well up [Settings]+[Sounds].

My final suggestion should for some reason, despite doing the above, your iPhone still appears to be a problem pay a visit to your nearest Apple store. If they confirm a problem, you don't have liquid damage and the phone is less than 12 months old, chances are they'll swap it for a new one.

Hope that helps and good luck!!