Some shows such as Highschool DxD have the title in their intro song in which you can hear what it stands for; by. So Kiss x Sis will be read as Kiss by Sis (it was either the opening or ending or throughout the show, but it was pronounced as kiss by sis, not sure where to find it). Highschool DxD will become Highschool D by D (in the song you can hear D by D). However this isn't final, based on other shows x can hold multiple meanings which become for example and or versus or just the multiplication. Some examples to illustrate what I am talking about:
- Romeo x Juliet -> Romeo and Juliet
- C³ Cube × Cursed × Curious -> C³ Cube and Cursed as well as Curious
- Hidamari Sketch × 365 -> Hidamari Sketch times 365
- Servant × Service -> Servant and Service
- Street Fighter x Tekken -> Street Fighter versus Tekken
With Hunter × Hunter for example the × is never pronounced but can be seen as a symbol indicating Hunter versus (or and) Hunter which is extremely fitting to the show itself.
Plenty of shows also use different symbols in their title for example Steins;Gate, Choukadou Girl ⅙, .hack//Sign, Tokyo Ghoul √A, Saiki Kusuo no Ψ Nan. Based on that the symbols pretty much always have a meaning related to the show itself. So in conclusion whether x (or ×) is pronounced or not it always has a meaning that fits the show. It however doesn't have a solid standard meaning. Sometimes the meaning is a lot easier to decipher and in other occasions it becomes quite a journey to discover what it means. The titles I picked as examples all have meanings in which you can see that some are extremely straightforward (if you watched the shows) while others aren't.