I have a couple of sound ideas. You may not be able to implement all of them, but maybe a few ideas for the audio department (so that they don’t get bored )
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Radiation/Geiger sound
Example: https://freesound.org/data/previews/328/328381_4437257-lq.mp3 -
Racing car sound
Example: https://www.fesliyanstudios.com/soundeffects/2019-05-28/ExportsCarNonAlts/Sports-Car-Driving-Med-www.fesliyanstudios.com.mp3 -
"Announcer kill" sounds
Example: https://opengameart.org/sites/default/files/audio_preview/preview_6.ogg.mp3 -
Propeller plane sound
Example: https://freesound.org/data/previews/457/457013_7482766-lq.mp3 -
More Level Up sounds
Example: https://opengameart.org/sites/default/files/SoundPack01-Preview.mp3 -
Zombie Growling
Example: https://freesound.org/data/previews/171/171638_2827474-lq.mp3 -
Casino sounds (cards, chips, etc)
Example: https://opengameart.org/sites/default/files/casinoSoundsPreview.mp3 -
Ticking clock sound
Example: https://freesound.org/data/previews/495/495705_10430677-lq.mp3