As an American who's done some travel (and school) overseas, I've found tipping vs rounding up rather murky. I've learned to apolgize profusely when I might have left more than appropriate. Seems to work.

Also, if someone doesn't respond to your question, it's not that they can't hear you, more probably they either don't have English, or are uncomfortable using very rusty skills. It's on you to be able to ask basic questions in whatever the native language is.