This issue appears to have first surfaced in an update released by Sky in September;
cf. https://helpforum.sky.com/t5/Sky-Go-Apps/Sound-quality-on-sky-go-app/m-p/3071369
There has been at least one update released by Sky on the Amazon Appstore since then for Amazon Fire devices but the audio issue was not addressed/resolved... the mind boggles why Sky bother to release updates for download on the Amazon Appstore if the sound issue isn't resolved?
As far as i am aware there is no workaround other than plugging in earphones, for the most part that is not an option i can afford because more often than not i need to also hear my surrounding environment when i use Sky Go on my tablet.
Three months with no fix in sight for something Sky apparently broke in the first place is shocking.