Notifications too quiet

I just bought a B1-850 for my ageing mother, mainly for use with Skype. Unfortunately the notification volume is too low for her - i.e. she struggles to hear an incoming Skype call (or email message). Does anyone know of a way of increasing the volume of notifications (I've already selected the maximum levels in Settings but it's still not loud enough).

I've tried "Volume Booster" (SeattleApps) and Volume Booster Plus (Doda Games) but neither made any difference.

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