Thanks Kostya, You are right. I have tested and posted result on another thread on here. I think the issue is generally an Android one as I have explained below.
Thanks for your help and here is a copy below for others:
I have done a few tests regarding this issue and come to the following conclusion :
Issue:
1. Apps do not retain Ringer or Notification Tones after the App or Android OS is restarted hence displaying "No Sound" in the AquaMail main notification settings.
2. Ringer and Notification Tones on Memory Card or Imported into default folder always tend to reset when Android OS or the associated App is restarted.
Reason:
From the info I have gathered, this happens because the Android OS always loads the Memory Card upon restart and for some reason most Apps are not able to pick the tones from the Memory Card, hence the "Unknown" or "No Sound" status.
Solution:
To avoid this situation, you are advised to use system default Ringer and Notification Tones as they do not reproduce the issue. I have tested this myself over days and haven't had the issue even after restarting the phone.
Hopefully this helps other out there as the issue is definitely not an AquaMail issue but rather the Android OS itself.