It comes up (originally?) in Heike Monogatari, as well as various 時代劇 etc.

http://questionbox.jp.msn.com/qa756594.html

I'm guessing that what might be tripping you up is the grammar of 遠からん者は音にも聞け. If you're used to modern Japanese, you might mistake 遠からん for 遠からない or 遠からず, but the ん here is a sound change from む, a classical 助動詞, attached to the 未然形 conjugation of the classical adjective 遠し, far.

So,
遠からん = （遠しの未然形）遠から　＋　（仮定・婉曲助動詞）む

In this case, it's used to express a conditional expression - "If you are far away (or, ones who are far away), listen intently" as given in the answer above.

Here's another example for reference: 花の咲かむ時に行け。
In modern Japanese: もし花が咲いたなら、その時に行きなさい。
Or, 花が咲くような時に行きなさい。

The reference page might be helpful.

Old books are difficult to read correctly.

--------------------------------------------------
Note added at 10 hrs (2011-10-05 12:51:29 GMT)
--------------------------------------------------

The translation on the above page seems to be based on a different text of Heike Monogatari, which can be seen on:

http://etext.lib.virginia.edu/japanese/heike/heike.html