Many users have reported that when using Skype on Android tablets, for some reasons the sound volume through the built-in speakers are extremely soft (too low volume). Even when the system volume is set to maximum and other audio players are working perfectly fine with good volume, strangely Skype just simply remains very very soft.
Unlike its other platform versions, Skype's volume test and level control is severely lacking on Android.
Workaround: A workaround I've tested maybe also helpful to you: download an Equalizer from Android Market. Just go to the Market and type "Equalizer", and you'll find dozens of free equalizers for your use. One equalizer I've tried gives me a present of equalizing with a widget handy to on/off my present. Once various frequencies are boost up, Skype's voice through built-in speakers sound much better!!
Of course, the down side of adding an equalizer is that it tends to distort your music. So, having an on/off widget switch is good when you'd like to listen to your favorite music and songs, you'd not want the EQ to kill your favorites - turn the EQ off.
Hope that Skype will soon have an upgrade to fix this very annoying problem...