Each version of this integration matches a specific build of Unreal Engine 4. Here is what has changed between the 2018.1.5.6835.1218 and the 2018.1.6.6858.1242 release of the integration (in addition to upgrading to the new Unreal build).

Unreal Engine 4.19/4.20/4.21 - Wwise 2018.1.6.6858.1242

• WG-40782 Changed Blueprint's AkCallbackInfo to be pooled and created on demand.
• WG-41003 Removed "Structure member is not initialized properly" warnings.
• WG-41101 Reduced compile time by stopping use of monolithic headers.
• WG-41204 Fixed: An Unreal window hangs when running in headless mode and the Wwise Project path is empty.
• WG-41245 Fixed: In Wwise Demo Game, WAAPI subscriptions to boolean calls do not return a call failed message.
• WG-41259 Removed Wwise Motion from the Integration Demo.
• WG-41339 Fixed: Sequencer Event clips continuously retrigger when clip length exceeds Event duration.