I have a question about whether it’s obligatory to reply to the Salam (of Muslims, of course) of people in the following scenario’s, seeing as they aren’t exactly in-person but they’re still the greeting:

1. One-on-one chat messages online (WhatsApp, Messenger, etc…); Voice messages via these platforms (and if it’s necessary to reply back with a voice message or whether a text back will suffice)

2. Group chats.

3. Online videos (where a presenter/speaker may start/end with a Salam). In this scenario, sometimes comments may be possible, sometimes they may not, in which case it can be quite difficult to contact the speaker. My usual approach in either case is just to reply Salam back to the screen as if the speaker can hear me, is this valid? Saying it like this makes it sound pretty strange but oh well

It is obligatory to return the Salam if it is possible for the person to hear you or understand you through some indication. Thus it is not required for the tv or online videos