Wouldya settle for a definite maybe not?
I don't own a T50 or T60 and it doesn't seem like any current owners of either model have answered your question since July. Seems something of a trend that owners of either of the newer models aren't as forthcoming with solutions as T10 and T30 owners. Many times questions regarding T50/T60 go unanswered, with T10 and T30 owners finally piping up to try to help. Dunno why it is that way, but it is what it is.
I can tell you that those two models are extremely similar in features and user interface as the older T10 and T30 models, both of which I do own. Soooo ...all other things being similar then it's quite likely the answer is that T50/T60 have bookmarking only for Audible files and not .mp3 files.