In American English, the "l" is produced in "fault", as is the "r" in "world". I can understand that these may be less pronounced in British English. In the word "doubt", the "b" is not pronounced because it comes after another labial-type (lip) sound. In this word, the preceding sound is "oo" (similar to a sustained "w" sound), in which the lips are protruded. Because "b" is also a labial (lip) sound, it is omitted. Yet, the "b" remains present because of the Latin root from which the word stems: "dubius" (from which we get the word "dubious"). This, by the way, is also why we see a silent "b" in other words: lamb, limb, climb, numb, thumb, tomb, etc. The preceding "m" is a labial (lip) sound, so the "b" (also a labial) is omitted in pronunciation.

Hello Lothar, I don't know of a certain rule for that , but there might be one though. I hope someone else could help you there. However there are many words in English where some letters are not pronounced beside the 'b',where no certain rule applies such as: 'l' in "fault", 'r' in "world" etc....

German is very close to English in grammar and sentence construction. But, b is english is not from doubt pronounce. Some people try to use it as reference, or using it as a guide for pronouncing "b".