Why does my yamaha mm8 doesn't have any sound? I try the headphones and there's no sound there either.

check mm8 drivers, uninstall and reinstall again.

I'd suspect the power supply is dead (check it with an ohm meter), or that the internal fuse is blown (open it up and check).

Your pedal may be the wrong polarity for this keyboard. Different manufacturers wire differently. And some use trs 1/4" and others use ts 1/4". Contact Yamaha customer service to get the correct pedal for your instrument.

<Do you mean it only has sound out of the headphones when connected and not out of the speakers when the headphones are disconnected? If so, your problem may be as simple as a dirty contact in the headphone jack.

If you can get sound from the headphones, it may be that you need to have that jack replaced- there's a cutout on a headphone jack so that the sound to your speakers is off when headphones are in use. If you cannot get sound from either the headphones or speakers, it's probably an amplifier problem- which needs a tech. Is it under warranty?

Try the headphone jack... if there is sound there, then it is either the speaker amplifier or a loose cable. It would be rare for BOTH speakers to fail completely.

see

http://www2.yamaha.co.jp/manual/pdf/emi/english/synth/mm6_mm8_en_om_b0.pdf

page 50

Try headphones... possibly the switch contacts on the headphone jack which disconnect the speaker when headpones are plugged in. If you can hear via the headphones, check the switch contacts. Also look for a loose cable.

Contacts can oxidize when idle for a long time.

Since it is brand new, IMMEDIATELY exercise your warranty rights as the LCD will probably have to be replaced.

I assume you have MIDI out MM8 cabled to MIDI in MM6?

You also have to setup the MIDI to be emitted by the MM8 and define the MIDI channel(s) to be used at both ends.

I assume you are sending note on/off, etc. information and not Sysex which MAY NOT be compatible between these units.