Each version of this integration matches a specific build of Unreal Engine 4. Here is what has changed between the 2017.2.8.6698.1053 and the 2017.2.9.6726.1089 release of the integration (in addition to upgrading to the new Unreal build).

Unreal Engine 4.17/4.18/4.19/4.20/4.21 - Wwise 2017.2.9.6726.1089

• Added support for Unreal 4.21.
• WG-38420 Fixed: Crash when Sequencer AkAudioEvent section is garbage collected while playing.
• WG-39408 Wwise Installation Path can now be configured per user. Use Wwise User Settings in the Project Settings to edit the Wwise installation path.
• WG-40128 Fixed: File handle leak in the IO hook.