If the speaker /buzzer is waterproof, this helps. Then you only have to keep the circuit board dry. There are three options.
1. Conformal coating. You can dip your electronics in this solution and allow it to dry. After it is completely dry, you will have a sealed device. This will not work if you have an IC in a socket. The conformal coating will get between the pins and legs of the ICs and sockets and give you a poor connection.
2. Potting. This is done with an RTV which is made for use with electronics. Most RTV has chemicals which will corrode the components in electrical circuits. Again, the sockets will not work here. You can make a small form for your circuit, out the board in it, and fill it with the potting material. With this option, you must throw away the circuit if it ever needs repair. It would not be worth the time to dig down to the components.
3. Make a waterproof chamber in your case which keeps the electronics (circuit board only) dry. Then the abuse is only on the speaker or buzzer which makes the sound. You will still need a vent somewhere, otherwise it will sweat and you will have moisture in the chamber which will not get out. The vent should be on the bottom using the method I described earlier.
Hope these suggestions will help.
MP