Homebrew requires a copy of the DSP firmware to be present at sdmc:/3ds/dspfirm.cdc. This can be obtained by using Luma3DS - select "Miscellaneous options..." in the Rosalina menu then "Dump DSP firmware" from there. This file can be copied to Citra's virtual SD folder but a 0 byte file is sufficient for homebrew audio to work since Citra uses HLE.