Small audio files the decoder tests load as embedded resources. Made with ffmpeg 9.0.1.

Mono441Hz.ogg: 0.1 s of a 441 Hz sine at amplitude 0.5, mono, 44100 Hz.
  ffmpeg -f lavfi -i "aevalsrc=0.5*sin(2*PI*441*t):s=44100:d=0.1" -c:a libvorbis -q:a 3 -fflags +bitexact -flags:a +bitexact Mono441Hz.ogg

Stereo441HzLeftOnly.ogg: the same sine in the left channel and silence in the right.
  ffmpeg -f lavfi -i "aevalsrc=0.5*sin(2*PI*441*t)|0:s=44100:d=0.1" -c:a libvorbis -q:a 3 -fflags +bitexact -flags:a +bitexact Stereo441HzLeftOnly.ogg

Clicks.ogg: 6 s of a click every 0.37 s over a 220 Hz tone that comes and goes, mono, 44100 Hz.  The clicks make the
encoder switch between short and long blocks.  Pages are 0.2 s long, so the file has 28 of them to seek between.
  ffmpeg -f lavfi -i "aevalsrc='0.8*lt(mod(t,0.37),0.002)+0.3*sin(2*PI*220*t)*lt(mod(t,1.1),0.5)':s=44100:d=6" -c:a libvorbis -q:a 0 -page_duration 200000 -fflags +bitexact -flags:a +bitexact Clicks.ogg

BigHeader.ogg: 0.05 s of a 441 Hz sine, mono, 44100 Hz, with a 70000 byte comment.  The headers run past the first 64 KB
a stream reads when it opens.  The comment comes from an ffmetadata file holding comment= and 70000 x characters:
  ffmpeg -f lavfi -i "aevalsrc=0.5*sin(2*PI*441*t):s=44100:d=0.05" -f ffmetadata -i bigheader.txt -map_metadata 1 -c:a libvorbis -q:a 3 -fflags +bitexact -flags:a +bitexact BigHeader.ogg

OpusInOgg.ogg: the first Ogg page of an Opus file, 47 bytes holding the OpusHead packet.  The decoder refuses it
there, and nothing later in a real Opus file is read.  It is the start of this file, cut after its first page:
  ffmpeg -f lavfi -i "aevalsrc=0.5*sin(2*PI*441*t):s=48000:d=0.1" -c:a libopus -fflags +bitexact -flags:a +bitexact OpusInOgg.ogg

Mp3WithoutTag.mp3: the first 64 bytes of an MP3 with no ID3 tag, so the file starts with a bare frame header.  64 bytes
is as much as the format sniffer reads, and the decoder refuses the file on that alone.  It is the start of this file:
  ffmpeg -f lavfi -i "aevalsrc=0.5*sin(2*PI*441*t):s=44100:d=0.1" -c:a libmp3lame -id3v2_version 0 -write_xing 0 Mp3WithoutTag.mp3