The voice only says "Hij" when you select the option to play the sentence slower.
The normal-speed version of this sentence is wrong, and needs to be re-recorded.
i think the audio says "Hij heeft de muis", not "een muis"!
It is so easy12³45800
Is muis used for the computer mouse as well?
Yes. Also "computermuis" if it isn't already obvious from context.
I definitely hear "de" and not "een".
why heeft and not hebt
It's because hebben (to have) is irregular:
Ik heb, jij hebt, hij/zij heeft, wij (and all plurals): hebben.