The sets 60948 and 60949 are suitable for ER 20 (Hercules), TRAXX, and Ludmilla locos from Märklin 36xxx hobby range.
The decoder from 60948 comes with pre-installed ER 20 sound, the set 60949 comes with pre-installed TRAXX sound.
Other sounds can be uploaded with a CS2.
The sets contain four parts:
Speaker, speaker bracket, new PCB for the loco, sound decoder.

This is the loco (TRAXX) in the original state:

You have to unsolder three wires (two brown, one red) and the two connectors of the motor.
The flex connectors for the lights can simply be pulled out of the sockets:

After unsoldering the three wires, you can remove the four screws and take the old decoder PCB out (don't lose the plastic toothpicks).

After unsoldering the motor:

Note the adhesive tape on the motor.
The new board moved onto the motor.

Note: I screw the new board into place before soldering the connections to the motor.
Board and plastic toothpicks (aka cardan shafts) back in place, ready to fasten the screws.

Here the connections and the soldering is done:

Here flex connectors and decoder are also in place:

Now push the two white wires through the hole in the frame:

Speaker installed and white wires connected:

The conversion is rather simple. The loco will keep the original (yellow) LEDs.
Is it worth it? The RRP for the sound sets is Euro 99.99, but sometimes you can get them for about Euro 65.
Only you can answer if it's worth it for you ...
Update: With the new mfx decoder the loco runs worse than with the original fx decoder. I won't do any more conversions with these sets. A conversion with ESU LokSound and white LEDs from Schönwitz doubles the costs, but loco looks much better and performs much better.
Update 2: The best settings I found so far for these decoders in those locos are motor type "c90" and K parameter set to 32 instead of the factory default of 64.
Motor runs more quiet than with the original fx decoder, probably due to the higher PWM frequency. Loco starts moving with a jerk that I cannot seem to get rid of.
The three-pole Mabuchi motor in those locos surely ain't no c90 motor. But it seems with the settings "DC soft" and "DC hard" the decoder tries to adjust to the motor "on the fly" and the loco speed at speed step 1 varies a lot. In c90 mode the decoder behaves in a deterministic way.
Edited by user 18 December 2015 08:57:44(UTC)
| Reason: Update: Added warning about worse performance after conversion. Update 2: Found improved settings.