Just a status update. Nothing I've tried seemed to help, the sound in Primo stays lower than the sound outside Primo whatever you do. The difference is smaller with build-in speakers and major using any external speakers either analog (via audio out) or blue-tooth which is bad because the small 3W BT speaker I tried has much better sound (outside Primo) than internal ones.
What I've ended up doing is applying a sound patch from here:
[Please Register or Login to download file] If you set the parameters properly (tried Eizo ) you get bot an improvement in loudness and in sound quality (so it's not equalizers only). This works inside Primo also so now internal speakers are usable at volume set to maximum for directions. There is still a difference, outside Primo max volume is already to loud you need to lower the volume, but the sound is usable on Primo for directions.
I could not find the explanation, it looks like both related to the software itself (it's lower inside Primo) but also related to the version of Android you use. Also it is different from device to device, on my phone, there is a difference in sound with external speakers but it looks to be much smaller than on Nexus 7.