To add a recording to go with a slide in Google Slides, you will actually be recording a video and inserting it on that slide. It will be easier to create a separate video for each slide so that it will autoplay when people play your slides. The slides will have to be in Presentation Mode for this to work when the person views it.
Creating your videos
To record your voice for each slide, you will be recording a screencast of your slide and then uploading that video to your Google Drive.
You can also create videos directly in your Drive using an app, directions can be found at https://www.youtube.com/watch?v=OmN4s8md8JE
Adding Videos to Google Slides
You should have a video for each slide in your slide deck. You will be inserting the video onto the slide and making it so that the video will autoplay when the slide starts.
Continue this for each slide, making sure to change the viewing permissions for each video so that anyone with the link can view.
Bonus tip: Change the sharing URL so it starts in presentation mode.
When you get the URL to share your presentation with others, the URL will automatically take them to where they are viewing your presentation but it is not in presentation mode.
And that URL can look like https://docs.google.com/presentation/d/1UUgSUqixAI5mzJLRQ7C59zpvivyHsSBIeueb4LA2Xx8/edit?usp=sharing
Delete the end of that URL until the slash before edit and add the word: present
Now when people click on that link, it will open up into presentation mode and look a lot better.