What's the rest of that song that goes "The horn, the horn, it sounds so forlorn"?

Search: the horn so forlorn

Why: You know, the Christmas jam in You've Got Mail.

Answer: Those maybe aren't the real lyrics.

It's called "The Instrument Song" or "The Orchestra"! (I also see it being called "Zur Feier," which means "to celebrate.") It appears to be originally Austrian or German, possibly by Willy Geisler in 1886, and there are a bunch of versions. Here they are all mashed together:

Violins:
The violins ringing like lovely singing
The violins ringing like lovely song

Clarinets:
The clarinet, the clarinet
Goes doodle doodle doodle det
The clarinet, the clarinet
Goes doodly doodly doodly det

Horns:
The horn, the horn
Awakes me at morn
The horn, the horn
It sounds so forlorn

Trumpet:
The trumpet is sounding ta-ta-ta-ta-ta-te-ta,
Ta-ta-ta-ta-ta-te-ta
The trumpet is sounding, ta-ta-ta-ta-ta-te-ta,
Ta-ta-ta-ta

Drums:
The drum's playing two tones
And always the same tones
Five one, one five,
Five five five five one

Source: Toy Town Germany, Songs for Teaching, Manhattan School

The More You Know: Here's a relatively clear version: