Audio guide

MPEG LAYER 3 AUDIO (.mp3)
Most common audio format for just about everything online
Format used for SoundCloud and Amazon Music store
High Quality: 320 kbps (2.5 MB per minute)
Low Quality: 128 kbps (1 MB per minute)

<format>
    <output>mp3</output>
    <audio_bitrate>128k</audio_bitrate>
    <audio_sample_rate>44100</audio_sample_rate> 
    <audio_channels_number>2</audio_channels_number> 
    <audio_codec>libmp3lame</audio_codec> 
    <file_extension>mp3</file_extension>
</format>

WIFI AAC (.m4a)
Dolby AAC-LC (Low Complexity) format for iTunes, YouTube, Vimeo
High Quality: 256 kbps (1.9 MB per minute)
Low Quality: 128 kbps (1 MB per minute)

<format>
    <output>m4a</output>
    <audio_bitrate>128k</audio_bitrate>
    <audio_sample_rate>44100</audio_sample_rate> 
    <audio_channels_number>2</audio_channels_number> 
    <audio_codec>dolby_aac</audio_codec>
    <file_extension>m4a</file_extension> 
</format>

MOBILE AAC (.m4a)
Dolby HE-AAC (High Efficiency) low bitrate format for mobile phones
64 kbps stream (0.5 MB per minute)

<format>
    <output>m4a</output>
    <audio_bitrate>64k</audio_bitrate>
    <audio_sample_rate>44100</audio_sample_rate> 
    <audio_channels_number>2</audio_channels_number> 
    <audio_codec>dolby_heaac</audio_codec>
    <file_extension>m4a</file_extension> 
</format>

OGG AUDIO (.ogg)
Open source audio format using Vorbis codec
High Quality: 320 kbps (2.5 MB per minute)
Low Quality: 128 kbps (1 MB per minute)
Better quality than MP3. Supported by Chrome, Firefox, and Android

<format>
    <output>ogg</output>
    <audio_bitrate>128k</audio_bitrate>
    <audio_sample_rate>44100</audio_sample_rate>
    <audio_channels_number>2</audio_channels_number>
    <audio_codec>libvorbis</audio_codec>
    <file_extension>ogg</file_extension>
</format>

DOLBY 5.1 SURROUND (.ac3)
Professional mix for theaters and DVD/Blu-Ray
High Quality: 448 kbps (3.4 MB per minute)
Low Quality: 384 kbps (2.9 MB per minute)

<format>
    <output>ac3</output>
    <audio_bitrate>384k</audio_bitrate>
    <audio_sample_rate>48000</audio_sample_rate> 
    <audio_channels_number>6</audio_channels_number> 
    <audio_codec>ac3</audio_codec>
    <file_extension>ac3</file_extension> 
</format>