Are the first two letters of "jung" and "jünger" sometimes pronounced the same?

Question

According to the letters the sounds in the beginning of "jung" is [ju] and in the beginning of "jünger" is [jy].

While is there synaeresis (even "j" is regarded as a consonant) combining [ju] or [jy] into the same vowel [y]?

And are the vowels got loose in these words, so we get [ø]?

Answer 1

German has a relatively strict mapping between sounds and letters. You can find this mapping-rules in this document, in § 1: Regeln und Wörterverzeichnis

Some of these rules are:

• The short vowels [ʊ] and [u] and the long vowel [u:] are written with the letter »u«
  ([u:] sometimes also as »uh«)
• The short vowels [ʏ] and [y] and the long vowel [y:] are written with the letter »ü«
  ([y:] sometimes also as »üh«)
• The short vowels [œ] and [ø] and the long vowel [ø:] are written with the letter »ö«

Examples:

• Mund = [mʊnt]
• zuvor = [ʦuˈfoːɐ̯]
• gut = [ɡuːt]
• Kuh = [kuː]
• hübsch = [hʏpʃ]
• Büro = [byˈʀoː]
• trüb = [tʀyːp]
• kühl = [kyːl]
• löschen = [ˈlœʃn̩]
• Ökonomie = [ˌøkonoˈmiː]
• schön = [ʃøːn]

Conclusions of this rules are (for German words only):

• The letter u is never pronounced as [ʏ], [y], [y:], [œ], [ø] or [ø:].
• The letter ü is never pronounced as [ʊ], [u], [u:], [œ], [ø] or [ø:].
• The letter ö is never pronounced as [ʊ], [u], [u:], [ʏ], [y] or [y:].

Note, that this rules apply only for German words. There are exceptions for foreign words like »Buffet« = [bʏˈfeː] which are pronounced very similar to the pronunciation in their original language (French in case of Buffet).

Jung is a German word, which means, that it obeys these rules. So, the pronunciation of jung and jünger are:

• jung = [jʊŋ]
• jünger = [ˈjʏŋɐ]

Note, that in German there also exist some minimal pairs for [ʊ]-[ʏ]

• Mutter - Mütter (mother, mothers)

  [ˈmʊtɐ] - [ˈmʏtɐ]

• drucken - drücken (print, press)

  [ˈdʀʊkn̩] - [ˈdʀʏkn̩]

I have heart some northern dialects, where the letter »i« sometimes is pronounced as [ʏ] instead of [ɪ] (»Fische« = [fʏʃə] instead of [fɪʃə]), but I'm not aware of any German dialect, where »u« is pronounced like »ü« or vice versa. And I also don't know any German dialect, where »u« or »ü« would sound like »ö«.

In German there also is no synaeresis that would eliminate the consonant [j] before any vowel.

Answer 2

I have never heard of ju and jü pronounced the same. (I am not native, but live here for a while) In German language the written form and the pronounced sounds have a clear and strong connection. You might have heard somebody speaking in dialect.