add custom audio mixer into audio decoder

i would like to see custom matrix for audio mixer in LAV filter audio decoder. i am very used to it from ffdshow and thats why i still use audio decoder from ffdshow and splitter and video decoder from LAV filter. 
![bez_nazvu](https://cloud.githubusercontent.com/assets/7086605/2544536/221fb6d8-b602-11e3-94c4-9ecd5734af9e.png)