Hmm, you most likely have one of the "first runs" of tama connections. Did it come with an extra battery in the package? If so, and all the batteries you've tried last about a week, take it back to the store you bought it from and exchange it for one that works better. Generally, it seems that the connections that DON'T come with the extra battery have significantly better battery useage.
well i got my tamagotchi a long time ago and haveant had to change the bats. since! they should last a really long time plus i usually keep the sound off on mine like at school and stuff