Normally custom sounds in versions of Minecraft before resource packs are impossible because Minecraft automatically downloads and overwrites default sound assets upon startup. This mod fixes that issue.
The Meaning of Life, the Universe, and Everything.
As somebody playing 1.7.3 from time to time I actually take this mod for helpful and like the fact that someone actually adressed this issue. 1.7.3 simply has the best worldgen of all versions (except for whats possible nowadays with the Customized one (which was not made by mojang)), and I remember once trying to make some kind of horror mc client with 1.7.3 but always had to disconnect from the internet in order to prevent mc from changing the files back to the original ones. I am not sure if I am actually going to use this, because just because I still play that version doesnt mean I still want to work with it towards other directions, but I am glad it exists. Good job.
This does not work. The launcher always replaces the jar and resources. If you want the old sounds, you have to load the game and then replace the resources folder. Please remove your post to stop wasting others time.
You can still use this mod (and any other mod that requires manual installation, as well as with automated installers which no longer install them properly) with the current launcher (also, since the 1.6 launcher, released in 2013, it would redownload a modded jar unless you followed the following steps, minus the downloads part, which was added in early 2016).
First, you have to make a copy of the "versions\b1.7.3" folder, then rename it to something else, as well as both the jar and json files inside (all to the same name). Then, open the json and delete the entire "downloads" section which includes "client.jar" (no quotes here), and also edit the following "id": "b1.7.3" to match the same name you used to rename the files. This will create a custom version which the launcher will ignore (you'll see a warning that it was unable to verify the file but it will run it).
Here is an example of what to edit in the json, using 1.6.4 as an example, but all the jsons that I've seen have the same general format (Beta 1.7.3 doesn't have an official server jar so it probably only has the client listed in the downloads):