iPod Hi-Fi's batteries don't fit perfectly and on batteries it's more quiet

My iPod Hi-Fi's batteries sometimes pop out after inserting them. They seem not to fit perfectly (and yes, I'm using the right ones). Is that normal and can anything be done against that problem?

Another problem I have is that if it runs on batteries, the Hi-Fi is by far not as loud as it is plugged in. It's even very, very quiet. Is this caused by the non-fitting batteries or is there another problem?