My guess would be that your lcd is damaged. Try taking off the back cover, and removing the metal plate in the top right hand corner that covers the ribbon cable connectors. Dis-connect then reconnect the lcd connector and try it. If that doesnt work, replace it.
The external speaker is soldered on to the motherboard. One of the wires might have pulled out of the solder when you took out the motherboard to put the part in. It has happened to me. Try checking the connections under the motherboard for the wires.
http://www.smartphone-parts.com/iPhone-4-Spare-Parts/?page=4 The part is only $3 here. I reccomend buying other parts too becuase it is $25 shipping. Its good for buying lots of parts at once on because this website has cheap high quality parts! You have to make an account to veiw prices though. But you need to buy the speaker and replace it, which is really easy
The problem will not be the lcd, as all that does is show the picture. This is something to do with the digitizer, or possibly the ipod software. I have three possible solutions: 1. Restore the ipod. The software may be acting up. But I dont think this will fix your problem. 2. The digitizer connector on the motherboard has been damaged. This can be replaced with some intense soldering skills. 3. You keep receving bad digitizers. Im thinking that your problem will be #2. I cannot tell you forsure though. In all honesty, the second generation can be purchased for a measly $30 if you go to the right places, so you might just have to get a new one. Or even upgrade. A good place to buy parts as I have found is www.smartphone-parts.com . Digitizers for the ipod touch 2 are only like $4. The shipping is around $25 for 4-7 day but if you are buying alot of parts its a good source. I hope this helped!