I have 3 audio sources, just one of these audio sources can't be heard on iOS without headphones, the others can clearly be heard through the device speakers. Why is this happening?

Riddle me this...

I have 3 audio sources.

Audio source 1 plays a clip formatted to native (wav) and set to load into memory.

Audio source 2 plays a clip formatted to native (wav) and set to load into memory.

Audio source 3 plays a clip formatted to compressed (mpeg) and set to decompress on load.

All audio sources have 3d sounds and the same initial settings.

Audio source 1 constantly plays with its volume and pitch varied as play progresses. This is successfully heard in unity and on device.

Audio source 2 constantly plays with its volume set to 0 until certain criteria are met and its volume is set to 1, it is intended to be heard "over the top of" audio source 1. This is successfully heard in unity but not on the device UNLESS you plug headphones in.

Audio source 3 constantly plays with its volume set to 0 until certain criteria are met and its volume is set to 1 (never occurs at the same time as 2), it is intended to be heard "over the top of" audio source 1. This is successfully heard in unity and on device.

To summarise I can hear some of the game's audio via my iPad's speakers but not Audio Source 2, unless I plug in headphones. I can hear all three exactly as intended when testing in unity. I don't think this is due to the app audio being set to mute or anything.

Any help would be greatly appreciated, in the meantime I'll be sporadically changing settings and/or approaches in the hope i'll stumble on a solution, I'll keep this thread updated as I progress.