Q:

In Asterix and The Magic Carpet, why does Cacofonix's singing start producing rain?

In Asterix and the Magic Carpet, Cacofonix's singing produces rain. (Previously, this had never happened.)
Why is it that when Cacofonix sings, it rains in #28 (Magic Carpet, 1987), but not in Books #1-27 (1961-83)? (Others may disagree, but I find this problematic, puzzling, and in need of explanation.)
(This effect also continues in the next book Asterix and the Secret Weapon.)

Four of the five current answers simply quote from Wikipedia's (unsourced) assertion that there is a French saying that when one sings poorly, it rains.
This could very well be an explanation. But absent either in-universe (i.e. within the comic books, perhaps cleared up in later books) or out-of-universe (e.g. interviews with Uderzo) evidence, this is just speculation. Moreover, it is unsatisfying because it fails to address my above question (in bold).

Edit: In case it isn't clear, I am not disputing the existence of such a French saying. I am disputing that the existence of such a French saying is a satisfactory explanation to my question. In particular, it fails to explain why Cacofonix's singing had not previously produced rain in the previous 27 books.

A:

This is an ongoing joke that Cacofonix's singing is so bad, it causes negative things to happen (a common trope). 

According to Wikipedia (My emphasis):

In Asterix and the Normans [his singing] is so unbearable that it teaches the fearless Normans the meaning of fear. In later albums his music is so spectacularly horrible that it actually starts thunderstorms (even indoors), because of an old French saying that bad singing causes rain.

The joke even leads to him causing it to rain inside:

And sometimes even leads to Cacofonix saving the day:

A:

According to the Wikipedia section on Cacofonix:

In later albums his music is so spectacularly horrible that it actually starts thunderstorms (even indoors), because of an old French saying that bad singing causes rain.

A French person reading the comics would automatically get the reference, and thus it isn't stated.  

A:

On page 6 of Asterix and the Magic Carpet (in-panel numbering), Vitalstatistix says regarding Cacofonix’ rainmaking abilities:

Oh, yes … I was forgetting, Cacofonix has new string to his lyre these days.

As noted by Darren, this is probably a pun involving adding a string to one’s bow, which analogously works with avoir plus d’une corde à son arc in the French original.
Now, there are two ways to interpret this:

The pun only happens out-of-universe, in which case the new string actually is responsible in-universe.
The pun happens in-universe, i.e., Vitalstatistix (as opposed to only Uderzo) makes the pun, in which case Cacofonix somehow acquired a new “skill” to do this.

Either way, the new rainmaking effects of Cacofonix’ music are addressed on the page: Either it’s because he actually upgraded (or rather: downgraded) his instrument or his abilities.

