What should I do with the remaining money that belongs to a woman who used to work for me and has passed away, knowing that she told me, "It's halal for me [to give it to you]," and she instructed me not to give it to her siblings? Is this money considered an inheritance, or should I give it as charity, especially since I bought a loudspeaker for the mosque with some of it?

All property, belongings, and debts left by the deceased belong to their legal heirs, and are to be divided among them according to the Quran and Sunnah. The deceased is permitted to bequeath only one-third of their estate to non-heirs. In your case, if the deceased genuinely told you that her money would be lawful for you after her death, this constitutes a bequest by which you are entitled to one-third of her estate. If the loudspeaker you took is equal to or less than one-third of her estate, then you owe nothing; rather, you may even take more from the estate until it reaches one-third. However, if the price of the loudspeaker exceeds one-third, you must return the excess to the estate, knowing that the aforementioned one-third is calculated from what remains after deducting the expenses for preparing the deceased for burial. This ruling applies if the heirs do not approve of what their deceased relative did; however, if they do approve, then it is enforceable.