Outlook plays a chime to warn you for scheduled reminders about emails, tasks, or calendar events. Unlike the new email alert (which you manage in Windows), you manage Outlook reminder, where you can change the time to something else or turn it off completely. Here's how to do it.
Turn off the clock
If you do not want a hearing alert every time a reminder pops up (which may be especially affected by headphones), then tell Outlook to not play a sound at all when a reminder is due . Go to File> Options> Advanced and scroll down to the "Reminders" section. The setting you are looking for is "Play Reminder Sound."
Turn off and then click "OK". Outlook will no longer play a sound when a reminder pops up.
Change Chime to Anything  If you still want a hearing alert, but you want something different than the usual "reminder.wav" clock, leave the "Play Reminder Sound" option and click "Browse" button next to it.
You can then choose any WAV file on your system to be used for your reminder alert. When done click on "OK" to exit. You do not have to restart Outlook, and your new audio will be played when a reminder appears.
If you have another file type you want to use, you must convert it (we are major Audacity and VLC fans for audio work, but use your own tool). If you want one of the other Windows sounds that you can access via Control Panel> Sound, find them on C: Windows media.