A Soundcache is a file with contains the first .125 seconds (125 milliseconds) of each sound required in a map. At level load, the Source engine loads certain sound .cache files instead of the full .wavs themselves in order to save memory and load time. During gameplay when a sound is requested, the sound begins to play immediately (consuming the 125 milliseconds of preload data in the .cache files) while the full sound is asynchronously loaded from the hard drive. In most cases, there is enough data loaded by the time the sound preload data is exhausted such that the async i/o never needs to "block" to finish loading. If the async layer, however, does need to block, then a noticeable framerate hitch can occur. These hitches can be displayed to the developer console by enabling the cvar
Re-building the .manifest Files
These instructions apply to MODs trying to generate the best possible .cache files:
1. Create a text file called maplist.txt and place it into your game directory. For example, in the case of Counter-Strike, it would live in the \cstrike directory.
2. Add in all of the map names to maplist.txt. Just add the map names without .bsp on the end of them (e.g. d1_trainstation_01, not d1_trainstation_01.bsp). Separate each map name with a newline.
3. Create a new .bat file and copy in this line:
steam.exe -applaunch nnn -w 640 -sw -console -condebug -nocrashdialog -makereslists -usereslistfile maplist.txt +mat_picmip 2 +r_lod 3 -autoconfig %1 %2 %3 %4 %5
nnn is the appropriate steam App Id for the game which you are modding.
4. Save and run the batch file. Running this process can take a very long time and if the engine crashes at any point then the data will not be valid. The engine must make it through the entire maplist.txt file cleanly to generate valid .cache files.
The .bat file will run the engine and start each map, wait a few seconds for things to settle, and write out a reslist to the \<gamedir>\reslists folder. These reslists are then used to build the actual sound .cache files. At exit, after running with the above command line, the engine should rebuild the meta-cache files and all map specific cache files based on the maplist.txt file.
mapname.cache is built using the accompanying .manifest files, which contains a list of sounds Precached by that map, as well as the reslist files mentioned above which contain the names of all files accessed during level load, not just those mentioned as Precached.
- .cache files can be viewed with Game Extractor 
- .manifest files can be viewed with any text editor
_sharedprecache.cache: contains data for precached sounds (see the Precaching assets section for info about precaching) common to over 50% of maps (e.g., sounds referenced by the response system which can occor in any map are always precached).
_other.cache: contains the file header, but no preload data, for any sound files that were not encountered in any other precache lists. Sounds in this cache will never cause I/O-related blocking as they play back fully asynchronously, that is, they only begin to mix data to the sound hardware after the sound data has completed asynchronous loading.
<mapname>.cache: Contains the remaining files that were Precached by the engine for the specified .bsp, but are not already a part of
_other_rebuild.cache: This is a temporary file used as a way to quickly rebuild the runtime .cache files above.
- Writes the .manifest file. Possibly buggy and probably not necessary based on using the above-described command line to automatically rebuild the .cache files.
- Rebuilds the .cache files. Only has effect if valid reslists and .manifest files exist.
Limitations & Bugs
The contents of the .cache files could be biased by hand-editing the .manifest files and requesting the engine to
snd_rebuildaudiocache. However, this is generally not advised.
Note: I haven't actually run this from the Steam cmd line, so if anyone runs into issues with the command line instructions, feel free to email me at email@example.com