Go to your device Settings, Apps, See all apps, tap on K-9 Mail.
Then tap on Notifications and you’ll see a list of all your accounts and the options. Longpress on Messages - Advanced - Sound, there you can select any notification/audio file from your device. (Actual steps may vary depending on your device manufacturer, but on my OP7 that’s how to get there). The lightblue edits in my screenshot are the different email account names.
Repeat that for every email account and you can select seperate notifications for each one.
