Cross laminated timber (CLT) is a modern building material which is gaining increasing application in Australian and New Zealand building developments. It is used as a structural wall and floor element, and has certain advantages over traditional building methods utilising concrete floors, particularly relating to the speed with which CLT structures can be constructed on-site. One disadvantage of CLT is that the base timber floor panel has less mass per square meter than concrete. Therefore, CLT typically requires additional layers of material (ceilings and raised floors) to achieve airborne and impact sound insulation performance similar to that of concrete floors and, indeed, to meet Building Code requirements. This paper explores a range of on-site test results obtained from two similar CLT apartment buildings. Standard airborne and impact sound insulation results (DnT,w, FSTC, LnT,w and FIIC) are presented, as well as heavy impact results obtained using a “Japanese ball drop” method (LiA,Fmax) to assess the low-frequency performance of the CLT floors. Various flooring upgrades were tested with the aim of improving the sound insulation performance of the floors. Test results from other apartment buildings with a mixture of concrete floors and timber joist floors are also presented and compared to the CLT floor results.