MP3
MP3 (MPEG Audio Layer III) is a lossy-compressed audio file format used by all of Valve's engines, and is available in all first-party titles. While engine limitations prevent it from being used for everything that WAV files can be used for, its smaller file sizes make it suitable for non-looping music or incidental ambient sounds in a soundscape.
Warning:
- Using GoldSrc or Source 1's default implementation of MP3 in a commercial game requires paying for the Miles Sound System license. Consider implementing an alternative, such as Ogg Vorbis or minimp3.
Strata Source and 64-bit version of the 
Team Fortress 2 branch use minimp3 by default. - MP3s cannot be properly looped in
Source! Looping can be simulated by scripting replaying the sound via entity I/O or code scripting, but there will be a gap.
- In
Source 2, proper looping MP3s are possible (
only S&Box's tools can flag the VSND appropriately), but they can only loop in their entirety. - MP3s loop normally in
GoldSrc.
- In
Note:
- MP3s with
ID3 metadata will have that metadata in the soundcache (fixed in 
). Avoid including metadata such as album art to prevent unnecessary bloat. - Many MP3 encoders will place ~100ms of silence before the sound to allow the sound to load. As such, MP3s are ill-suited to time-sensitive applications, where WAV should be used instead. If compression is necessary, see Compression (Source 1)#Audio Compression.
Recommended minimum bitrates
Unlike WAV, MP3 supports arbitrary bitrates, independent of sample rate and channel count. While this improves scalability, going below certain bitrates will result in noticeable quality degradation. Below are recommended minimum bitrates for the sample rates supported by all versions of Source (some third party games, as well as Source 2, support additional sample rates).
| Sample rate | Mono (1 channel) | Stereo (2 channels) |
|---|---|---|
| 11025 Hz | 32 kbps | 64 kbps |
| 22050 Hz | ||
| 44100 Hz | 64 kbps | 128 kbps |
These are just minimums. If you have the space to spare, you increase the bitrate for higher quality audio! Setting the bitrate to double the values listed above will result in a less aggressive low-pass filter (at least in LAME).
The maximums for each sample rate are different, as MP3 uses slightly different codecs for each sample rate:
| Sample rate | Max bitrate | Codec |
|---|---|---|
| 11025 Hz | 64 kbps | MPEG 2.5 Audio Layer III |
| 22050 Hz | 160 kbps | MPEG 2 Audio Layer III |
| 44100 Hz | 320 kbps | MPEG 1 Audio Layer III |