Pingterval!

Choose the minute, set the volume, mute when you want silence.

This extension uses the browser's Alarms subsystem. This means it uses resources efficiently, but its Pings may be delayed slightly, most noticeably after the computer wakes from sleep mode. As such, it is not recommended for use in the management of critical infrastructure such as a nuclear weapons facility.

But if you just want to have a gentle notification of time passing as you focus on a task, you might find Pingterval! ideal.