The steps below outline a few ways to adjust the alert/ alarm volume on your iOS app.
If you're not hearing an audible alarm at all when timing is complete, please check out this guide to enable the alert.
In-App Time Alert settings
In the timer settings, there are Tone and Vibrate settings. Tap on that to open two sliders for Tone and Vibrate. When they are green, sound is ON.
Using the Volume Buttons or Sound Settings
The physical buttons on the side of your device are your main volume buttons: turn up the volume using the top button, and turn the down the volume using the bottom button. If this does not change the volume, navigate to Settings > Sounds and slide the Ringer and Alerts slider at the top of the page. You may also want to enable the "Change with Buttons" option to allow you to change the volume via the buttons on the side of your device.