I have heard it somewhere but I can not find any refference.
In each of the Philippics, Demosthenes rails against Philip's underhandedness and his fellow citizens' cowardice and complacency. References to money are generally confined to questions of raising funds for war and there is no reference to gold mining by Philip. The fourth Philippic talks the most about money, and in that, Demosthenes indicates that inflation is not a problem:
If therefore, noting the abundance and cheapness of goods for sale in your markets, you have been beguiled by these things into the belief that the city is in no danger, your estimate of the situation is contrary to all right and reason.
Never heard of the guy before. However, the wiki page you linked said:
Demosthenes grew interested in politics during his time as a logographer, and in 354 BC he gave his first public political speeches. He went on to devote his most productive years to opposing Macedon's expansion.
So basicaly, he was an anti-Macedonian political pundit.
You only have to watch a few minutes of this type of person today to realise that they will make any argument available that they think might advance their point. So if he noticed an inflationary effect, or even the hint of one, or even something that could be argued with a bit of work to make it look like there was one, he would have made that argument.
So, while it would be nice to find the actual reference, if nothing else we can say it certianly would have been in character for the guy.