How to adjust siri's volume so it will sound louder via earpod? This happens lately and I don't know how to fix it.

When I  listen to podcast  using the app 'cast' with my earpod, from time to time I long press the remote to ask for some information. Let's say the volume of the podcast output is at 8 (out of 10), the siri answer will out very weak and feels like it is set to 1 or 2. 

I have tried increasing the volume when siri is activated but it does not have any effect. 

Is there anything I can do to increase siri's volume?

EDIT Add model and version info

The iphone is 6s (NKQN2ZP/A)

OS version is 10.2.1
Siri has an independent volume. When Siri is talking, use the volume buttons on your iPhone to change the volume of Siri. This is preserved for next time Siri is used.