我觉得这个现象最适合的情况是 /l/ 加 后元音 /u/ 时，只有在这种情况下，/l/ 音可以省略
例如：
Revolution:
Volume;
Volunteer;

Rethinking/rewording on this:

/l/ sound can be ommited in some words but cannot be replaced with /r/ sound, although the sounding organ (tip of the mouth) is in the position of /r/ sound: