It didn't help much to be honest :embarrass: I just translated the sentence word by word. This is what I managed to do, I hope the sentence makes some sense to you.
"Rog in fagot s tremolom zvočil kot so: flavta na poteg, okarina, ustnik kljunaste flavte ali podobnimi sledita zaznačeni konfiguraciji. Pri tem naj zvočno ne preglasita ostalih."
Horn and bassoon follow the indicated configuration, with a tremolo of musical instruments such as "flavta na poteg", ocarina, the mouthpiece of a recorder or similar instruments. The sound of the horn and bassoon should not drown the rest.
I couldn't figure out what a "flavta na poteg" is. There are only four google hits. Three of them are lesson plans for preschool children. This instrument seems to be used in musical education which follows an Edgar Willems method? The fourth hit was the site where you found this sentence.
"Potegniti" means to pull something, "flavta na poteg" would be a flute where you have to pull something, if this tells you anything??
Here you can see a photo of a whistle "na poteg", maybe this will help:
http://www.lastracom.com/showimage.aspx?docid=6383&width=150&source=DD&ident=0910003&xpath=images/langI/records/record/image&pos=0&appid=LASTRACOM
(It's from a commercial site, but it's the only similar thing I could find, I hope the mods won't mind.)