Google, sadly, fails me on an image search for the sort of thing I'm talking about, so here's a quick example I doodled freehand while waiting for an FTP download to complete!
If you've read my Illustrator guide, you'll already know all about creating normal SFX, so we'll get through the basic process as quickly as possible…
Create your text as normal:
Convert to outlines, arrange to your satisfaction and merge:
Now, copy your SFX and use the Paste In Back function:
You now have an identical copy behind your original. It should still be selected but, if it's not, use the Selection tool to draw a marquee round the FX (which will select both versions) and then de-select the top one with SHIFT-Click.
With the back copy selected, you can increase the stroke weight so that the back copy's stroke becomes visible:
You can now break up the heavier stroke at the back using the 'Dashed Line' option:
Admittedly, this doesn't look terribly attractive to start with, so the next thing to do is get rid of those nasty hard corners on the dashes, by changing the Cap and Corner settings of the stroke:
Obviously, this is better, but still not great. You can use the Dash and Gap settings to get a much more pleasing result:
This is much closer to the effect we're looking for but — to my eye at least — still looks a little too regular and mechanical. Fortunately, we can very the settings for Dash and Gap by more than just a single set of parameters:
Which gives us a much better result. If you want an inline, as per the hand-drawn sample we started with, simply use the Offset Path function with a negative value:
…And you should have a splendidly old-school sound effect!