A simple audio-filter

I need to find a way to build a simple audio filter.
It's for an electronic talking toy which uses a piezo-speaker. I've
removed the speaker and tried to make a line-out instead, but the sound
is pretty "gritty" and "dirty". I believe I can make it better with
some adjustments (filtering)
This is what I've done so far:

220K 10nF
----------------->---------------o---xxxx----o o--||----->
| |
50K x |
audio Audio x<----| line
out Taper x out

I've experimented with different capacitors. If I lower the value (say
6 nF) the bass goes away (which includes some low frequency "rumble")
and I get more treble. If I insert a capacitor with a higher value (say
40 nF) the opposite happens.
I would like to remove the annoying "rumble" and "dirty" part of the
sound, but without removing all of the bass in the process. How do I do
this in the simplest possible way?