How do I change the audio prompts alert volume?

I have bluetooth headphones connected to my watch. I have enabled lap alerts for my device. When I run I get an alert for every lap which I can hear in my headphones. This is a fantastic feature however the volume is very low and it is difficult to hear. How do I increase the volume of the alerts?

The only way I have found to do this is to start playing some music, increase the volume on the watch and then stop playing the music however this is not something I want to do whilst racing.

My headphones are the Aftershokz Aeropex, they have a volume control on the side which increases the volume when connected to other devices (Apple Watch, iPhone, etc).