I was just watching Q I and heard it said that helium only changes the pitch of your voice because sound travels faster through helium - thus changing the pitch
Are you thinking what I'm thinking? Would putting helium in your melodeon say through the air button, then playing through the reeds make your instrument really squeaky for a short time?
If anyone has any inflated baloons around.....
No, because with free reed instruments the pitch is almost entirely determined by the stiffness and mass of the reed, and only slightly by the air (or helium) load. This is unlike voices and reed/woodwind instruments where the pitch is determined by a resonant column of air (or helium).
There would be a small change in pitch, probably a fraction of a semitone -- I remember a New Victory Band concert many years ago at Pontefract Town Hall where they had a dry ice machine next to Chris Coe's harmonium, and when they turned it on during a tune it sucked in the cold dense CO2 and went horribly flat...
Ian