Answer to: Why does my iPod only play through it's speaker?Start by cleaning out the headphone jack with a Q-Tip and some Isopropyl, just to be sure there is nothing jamming the headphone jack on. However, having been dropped the likelihood of a connection on the Logic Board having been dislodged is quite high. Your best bet is to follow the repair guide available on this site in order to confirm if this is the case. If so simply reconnecting the offending cable should resolve the issue. If not it's time to consider part or unit replacement.
Answer to: How do I find the model numberIf I understand the question I think your looking for the Model Identifier? If so, you will need to navigate to the menu in the upper left hand corner of the menubar. From there you will see the About This Mac... option, by holding down the alt key (left/right of spacebar) this will change to System Profiler... (Leopard + SnowLeopard only). Alternatively select About This Mac... and then click More Info (bottom,centre). Select System Profiler, this will open a new window listing different hardware and software attributes of your Mac. The very top menu, named Hardware Overview, will list Model Name and then Model Identifier e.g MacBook 5,2 etc. Hope that helps, if not we'll have to try again.