This is a problem with all phones. It doesn't matter which application you install to record, because the other party's sound volume will always be lower than yours. I recommend transferring those recording files to PC and then use an audio software to modify the output volume. For example, Audacity is capable of doing this. Install the software and then load the file. On the left side next to the file wave spectrum, there is a volume slider. Drag it to right and the volume will be amplified.
Download Audacity: http://audacity.software.informer.com/