The dll's listed must be included in the AviSynth 2.5\plugins folder.
Some files are included because are from the BeHappy team, other must be downloaded.
There are some bass*.dll than must be renamed to bass_*.dll to be load properly in AviSynth.

NicAudio.dll	To decode AC3, DTS, MPEG, LPCM, and other uncompressed formats (included)

BassAudio.dll   Interface to open Bass Audio libraries v2.4 in AviSynth (included)

bass.dll        Main bass library, to open WAVE_FORMAT_EXTENSIBLE, mp2, mp3, ogg, ...Link(1)
bass_aac.dll    Optional to decode AAC/MA4/MP4. Link(1)
bass_ape.dll    Optional to decode APE Monkey audio. Link(1)
bass_cda.dll    Optional to read CD Audio. Link(1)
bass_flac.dll   Optional to read Flac until v1.2.1. Link(1)
bass_mpc.dll    Optional to read MPC, MusePack. Link(1)
bass_spx.dll    Optional to read SPX, MusePack. Link(1)
bass_tta.dll    Optional to read TTA, True Audio encoder. Link(1)
bass_wma.dll    Optional to read WMA, Windows Media Audio. Link(1)
bass_wv.dll     Optional to read WV, WavPack Audio. Link(1)

soxfilter.dll   If you want use the Upmix DSP function (or for others many uses). Link (2)

(1) The oficial releases v2.4 are in http://www.un4seen.com/bass.html

(2) http://forum.doom9.org/showthread.php?p=761154#post761154

