Do you know what version his mac Mini is?
Do you know what type of processor is in it?
If not can you click the apple icon in the top left then choose about this mac and let me know what it says.
If you cannot get sound from the internal speaker on the Mini then it is possible you may ahve damaged the speaker connector wire which is apple part number(NNN) NNN-NNNNwhich you can see on this exploded diagram
If that is the case the only way to get yoru sound back would be to replace the speaker kit.
An easy way to test this would be to see if you plug a set of headphones into the plug on the back of the machine will it then give you those as an option for playback, If so then it is the speaker kit that is damaged.
Test that and let me know what you find out.ThanksEric
Just give the headpones a try and let me know if you then get those as an option.
Looking at the diagram do you recognise anything else as the part you may have damaged?
That is possible, However the issue is that without taking the machine apart again and re assembling it again there is no way to make sure everything is where it belongs.
Can you take a look at these screenshots from sterpin and let me know if you recognise the cable that may have been damaged http://www.sterpin.net/uk/ddMacMiniuk.htm
As with anything as long as you take your time and are careful you should be able to do it as long as the connectors are not damaged.
Just take a good look at the dissasembly instructions and if need be you may need to purchase a good set of needle nose pliers so that you can get to those hard to reach places.
If you need clearer screenshots or any help let me know