How to avoid a electrolytic capacitor on a (audio) signal path?
The input is virtual earth (It is basically the inverting input of an opamp buffer, so the time constant is defined by R1 or R6 depending on the channel. 

I would possibly use a non polar part there, but really the trick with electrolytic caps as DC blocks is to make them large, a cap forming a pole way below the audio band has by definition little signal voltage dropped across it and thus contributes negligible distortion. Obviously a cap forming a pole in a filter network (Something like a passive crossover) has issues, but for small signal DC block a non polar elco is generally fine. 

There is a LOT of crap talked about capacitors, and it IS possible to use the wrong ones in the wrong applications, but a non polar elco there will do just fine in practice. 

C. Bateman did the classic work on this stuff back in the day in a series of articles in E&amp;WW, well worth tracking down.