RMS levels for ACX acceptance

How do I increase my RMS 1.5dbs for my already recorded narrative in garageband to satisfy ACX? I have submitted all 14 chapters as individual files to ACX and all were accepted except for 5 chapters. In each one ACX said I needed to increase the RMS 1.5-2.0 dbs. How do I do that in the most straight forward manner? I already changed the garage band preferences to "export with full volume", which is how all the rest of the chapters got accepted. These five just came up a bit short.

Thanks so much!

BDall