On iOS and iPadOS, there are two separate volume settings that affect Timer+:
Timer+ sounds: the alerts you hear while the app is open.
System alert sounds: the notification sounds you hear when Timer+ is running in the background.
Adjusting Timer+ Sounds (while the app is open)
Start a timer and wait for it to go off, or preview a sound in the picker.
While the sound is playing, use your device’s volume buttons to raise or lower the volume.
Adjusting System Alert Sounds (background notifications)
Open the Settings app.
Tap Sounds & Haptics.
Under Ringtone and Alerts, use the slider to set your alert volume.
If Change with Buttons is enabled, you can also adjust this volume with the device buttons from the Home or Lock Screen.