Just take a look at Dark Miasma guides and Sonic Blast guides. That said, I had a Dark/Sonic build lying around. Hope you like it! 
Hero Plan by Mids' Hero Designer 1,401
http://www.cohplanner.com/
Click this DataLink to open the build!
Level 50 Magic Defender
Primary Power Set: Dark Miasma
Secondary Power Set: Sonic Attack
Power Pool: Fitness
Power Pool: Leaping
Power Pool: Speed
Ancillary Pool: Dark Mastery
Hero Profile:
Level 1: Twilight Grasp -- Nictus-Acc/EndRdx/Rchg(A), Nictus-Acc/EndRdx/Heal/HP/Regen(27), Nictus-Acc/Heal(27), Nictus-Heal(29), Nictus-Heal/HP/Regen/Rchg(29)
Level 1: Shriek -- Decim-Acc/Dmg/Rchg(A), Decim-Acc/EndRdx/Rchg(3), Decim-Dmg/Rchg(7), Decim-Dmg/EndRdx(7), Decim-Acc/Dmg(9)
Level 2: Scream -- Decim-Acc/Dmg(A), Decim-Dmg/EndRdx(3), Decim-Dmg/Rchg(5), Decim-Acc/EndRdx/Rchg(15), Decim-Acc/Dmg/Rchg(17)
Level 4: Tar Patch -- RechRdx-I(A), RechRdx-I(5)
Level 6: Hurdle -- Jump-I(A)
Level 8: Darkest Night -- DarkWD-Rchg/EndRdx(A), DarkWD-ToHitDeb/Rchg(9), DarkWD-ToHitDeb/EndRdx(11), DarkWD-ToHitdeb/Rchg/EndRdx(11)
Level 10: Combat Jumping -- LkGmblr-Rchg+(A), Zephyr-Travel(13), Zephyr-ResKB(13)
Level 12: Howl -- Range-I(A), Posi-Dmg/Rng(15), Posi-Dmg/Rchg(37), Det'tn-Acc/Dmg/EndRdx(39), Det'tn-Dmg/Rchg(39)
Level 14: Health -- Mrcl-Rcvry+(A)
Level 16: Shout -- Thundr-Acc/Dmg(A), Thundr-Dmg/EndRdx/Rchg(17), Thundr-Dmg/EndRdx(23), Thundr-Dmg/Rchg(25), Thundr-Acc/Dmg/Rchg(25), Thundr-Acc/Dmg/EndRdx(34)
Level 18: Fearsome Stare -- Cloud-ToHitDeb(A), Cloud-Acc/ToHitDeb(19), Cloud-Acc/Rchg(19), Cloud-ToHitDeb/EndRdx/Rchg(21), Cloud-Acc/EndRdx/Rchg(21), Cloud-%Dam(23)
Level 20: Stamina -- P'Shift-EndMod(A), P'Shift-EndMod/Rchg(31), EndMod-I(31)
Level 22: Amplify -- AdjTgt-ToHit(A), AdjTgt-ToHit/Rchg(40), RechRdx-I(48)
Level 24: Shadow Fall -- HO:Ribo(A), HO:Ribo(31), S'fstPrt-ResDam/Def+(34), LkGmblr-Def/EndRdx(39), LkGmblr-Def(40), LkGmblr-Rchg+(40)
Level 26: Hasten -- RechRdx-I(A), RechRdx-I(43), RechRdx-I(46)
Level 28: Super Jump -- Zephyr-Travel(A), Zephyr-Travel/EndRdx(37)
Level 30: Howling Twilight -- RechRdx-I(A), RechRdx-I(34)
Level 32: Dark Servant -- DarkWD-ToHitDeb(A), DarkWD-ToHitDeb/Rchg(33), DarkWD-ToHitDeb/EndRdx(33), DarkWD-Rchg/EndRdx(33)
Level 35: Screech -- Stpfy-Acc/Rchg(A), Stpfy-EndRdx/Stun(36), Stpfy-Acc/EndRdx(36), Stpfy-Stun/Rng(36), Stpfy-Acc/Stun/Rchg(37), Stpfy-KB%(42)
Level 38: Dreadful Wail -- Oblit-Dmg(A), Oblit-Acc/Rchg(42), Oblit-Dmg/Rchg(42), Oblit-Acc/Dmg/Rchg(43), Oblit-Acc/Dmg/EndRdx/Rchg(43)
Level 41: Oppressive Gloom -- Acc-I(A)
Level 44: Dark Consumption -- Efficacy-EndMod(A), Efficacy-EndMod/Rchg(45), Efficacy-EndMod/Acc/Rchg(45), Efficacy-Acc/Rchg(45), Efficacy-EndMod/Acc(46), Efficacy-EndMod/EndRdx(46)
Level 47: Soul Drain -- Oblit-Dmg(A), Oblit-Acc/Rchg(48), Oblit-Dmg/Rchg(48), Oblit-Acc/Dmg/Rchg(50), Oblit-Acc/Dmg/EndRdx/Rchg(50)
Level 49: Dark Embrace -- HO:Ribo(A), HO:Ribo(50)
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Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Empty(A)
Level 1: Vigilance
I've started a Dark/Sonic that I really like but was wondering if there were any guides around for guidance as it were.