It’s been a while since I’ve messed around with Google Slides.
Basically, you need an audio file saved locally—you know, those standard .mp3 or .wma files—but you might need to convert them to a .wav format first if things get finicky. You can just grab a free converter online for that. Once you have the file, you head over to the transitions section in Google Slides 😂—I’ve spent so many years using Google Workspace in English that seeing different terminology feels a bit surreal, like trying to read a foreign language in my own house—where you can select sounds. If you scroll down to the custom sound option, you can upload your .wav file there. As I recall, the track should keep playing through the transitions, though you might need to tweak a few settings to make sure it doesn't cut out between slides. Hopefully, that points you in the right direction.