Doombar Bob

I hvae just purchsed a 54 plaste RX300 SE-L, loving the car however, whilst all the audio appears to be working, no volume comes out. I have informed the garage I bought it off as fault was there from day one and they say they cant fix it and it is £2k to replace unit. Does anyone know of this fault and if it can be repaired without replacing the unit? Thanks Rob