Despite the fact that people want to believe that A12 [Mori No Kinoko, which translates to Mushroom Forest] is Beware the Forest Mushrooms from Super Mario RPG, I honestly don't thnk that is it because they would try to get licensing from Square before even considering it, which is why I believe it is the song
In the Mushroom Forest from the first Mario Party.
Also, here's a copy-and-paste of the "full" list of unused tracks in Brawl, note that Wild Trax is Stunt Race FX in the US (and was the first 3D home race game made by Nintendo) and the W22 is most likely Saria's Song because in a beta screenshot from Nintendo it was listed as a song to play on Hyrule Temple:
snd_bgm_X12_SIMPLEINTRO (Classic Mode Intro)
snd_bgm_X14_ENDING (Classic Mode Ending)
snd_bgm_X24_HOW2PLAY (How to Play)
snd_bgm_A11_MLRPG02 (Another piece from one of the Mario & Luigi RPGs)
snd_bgm_A12_MORINOKINOKO ("Mori no kinoko" = Mushroom Forest...which game? Could it be...SMRPG?!)
snd_bgm_C06_KAZENOSAKANA (Ballad of the Windfish)
snd_bgm_E04_COOKIE (Yoshi's Cookie)
snd_bgm_G06_COMMAND (Star Fox: Command)
snd_bgm_J01_STAGECHANGE (No idea...maybe BGM for when Castle Siege changes forms?)
snd_bgm_J05_ERABARESHI (Erabareshimono = "The Chosen One(s)")
snd_bgm_K02_SENTOUONIISAN (Battle with Masked Man from MOTHER 3)
snd_bgm_K03_EIGHTMELODIES (Eight Melodies from EarthBound)
snd_bgm_K04_SMILEANDTEARS (Smiles and Tears from EarthBound)
snd_bgm_K06_BECAUSE (Because I Love You from EarthBound)
snd_bgm_M14_WARIOSTAGE (Another Microgame$ tune)
snd_bgm_N04_RADIOTAISO ("Radio Taisou" = Radio Exercises)
snd_bgm_Q03_SPORTSMEDLEY (Soccer / Baseball / etc. on the NES, probably)
snd_bgm_R01_WILDTRACKS (No clue...)
snd_bgm_S01_MAINTHEME (MGS Main Theme...no doubt taken out of the main game because of the composition controversy)
snd_bgm_S09_BEATMANIA (MGS music from Beatmania?)
snd_bgm_T04_HOWTOPLAY (Original "How to Play" music?)
snd_bgm_U05_UCANDO ("You Can Do It"...? Probably another vocal track.)
snd_bgm_W22_DX55 (The missing Melee track...Fourside?)
snd_bgm_Y06_ADV06 (Missing Subspace Emissary music)
snd_bgm_Y12_ADV12 (Missing Subspace Emissary music)
snd_bgm_Y18_ADV18 (Missing Subspace Emissary music)
snd_bgm_Y19_ADV19 (Missing Subspace Emissary music)
snd_bgm_Y20_ADV20 (Missing Subspace Emissary music)
snd_bgm_Y21_ADV21 (Missing Subspace Emissary music)
snd_bgm_Y22_ADV22 (Missing Subspace Emissary music)
snd_bgm_Y23_ADV23 (Missing Subspace Emissary music)
snd_bgm_Y24_ADV24 (Missing Subspace Emissary music)
snd_bgm_Y25_ADV25 (Missing Subspace Emissary music)
snd_bgm_Y26_ADV26 (Missing Subspace Emissary music)
snd_bgm_Y27_ADV27 (Missing Subspace Emissary music)
snd_bgm_Y28_ADV28 (Missing Subspace Emissary music)
snd_bgm_Y29_ADV29 (Missing Subspace Emissary music)
snd_bgm_Y30_ADV30 (Missing Subspace Emissary music)
snd_bgm_Z09_FLUIGI (Luigi's Victory Theme)
snd_bgm_Z12_FKUOPA (Bowser's Victory Theme)
snd_bgm_Z13_FPEACH (Peach's Victory Theme)
snd_bgm_Z14_FZELDA (Zelda's Victory Theme)
snd_bgm_Z15_FSHEIK (Sheik's Victory Theme)
snd_bgm_Z19_FFALCO (Falco's Victory Theme)
snd_bgm_Z20_FGANON (Ganondorf's Victory Theme)
snd_bgm_Z24_FZEROSAMUS (ZSS's Victory Theme)
snd_bgm_Z26_FLUCAS (Lucas' Victory Theme)
snd_bgm_Z27_FDIDDY (Diddy's Victory Theme)
snd_bgm_Z28_FPOKETRAINER (Pokemon Trainer's Victory Theme)
snd_bgm_Z32_FDEDEDE (King Dedede's Victory Theme)
snd_bgm_Z33_FLICARIO (Lucario's Victory Theme)
snd_bgm_Z34_FIKE (Ike's Victory Theme)
snd_bgm_Z37_FPURIN (Jigglypuff's Victory Theme)
snd_bgm_Z38_FMYU2 (Mewtwo's Victory Theme)
snd_bgm_Z39_FROY (Roy's Victory Theme)
snd_bgm_Z41_FTOONLINK (Toon Link's Victory Theme)
snd_bgm_Z52_JCLEAR1 (Some accomplishment music...probably for clearing a stage / event)
snd_bgm_Z53_JCLEAR2 (More accomplishment music)