Digitrax (and certainly a number of other manufacturers) make a variety of different sound decoders, and Athearn makes a number of different RTR locos.
Whether a given sound decoder will be a good fit in a given RTR loco depends on which decoder and which loco.
The bigger question is probably how to fit in an appropriate speaker (or two) along with that decoder.
As far as I know, the Athearn RTR locos (as opposed to their Genesis series) have chassis with no ready provisions for adding a speaker. How to fit one in is therefore a matter of how much room you can find, or make, to accommodate the speaker(s) of your choosing.
Keep in mind, some Athearn RTR are equipped with DCC plugs and others are not. IIRC, the SD45T-2's, SD40T-2's, SD45's, SD40-2's etc. all have DDC ready plugs. The GP40-2 may need the motor isolated before adding DCC decoder, similarly to the blue box Athearns. So simply using circuit board replacement style decoders may not be all you need to do depending on the RTR loco. Unfortnately Athearn's RTR line is quite variable so the answer is "it depends".
Rio Grande. The Action Road - Focus 1977-1983
Soundtraxx, ESU, and QSI all have decoders of the form factor to replace the factory Athearn boards Most of the newer RTR models have a 9 in plug so you dont necessarily replace the board, but doing so nets you more room for the speaker and enclosure.
--Randy
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Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
They might make a sound decoder with appropriate sounds, but Soundtraxx makes decoders designed for Athearn locomotives. Soundtraxx is the OEM for Athearn's recent sound decoder equipped locomotives.