Labiovelars
Numerous additional changes affected the labiovelars.
Even before the operation of Grimm's law, they were reduced to plain velars in the vicinity of /u/. This appears to be a sound law that was inherited from PIE and continued to operate as a surface filter, i.e. if a sound change generated a new environment in which a labiovelar occurred near a /u/, it was immediately converted to a plain velar. This caused certain alternations in verb paradigms, such as *singwanaN [siŋɡʷɑnɑ̃] ('to sing') versus *sungun [suŋɡun] ('they sang').
After the operation of Verner's law, various changes conspired to almost completely eliminate voiced labiovelars. Initially, [gʷ] became [b], e.g. PIE *gʷʱédʱyeti > PGmc. bidiþi "(s)he asks for". The fricative variant [ɣʷ] (which occurred in most non-initial environments) usually became [w], but sometimes instead turned into [ɣ]. The only environment in which a voiced labiovelar remained was after a nasal, e.g. in *singwanaN [siŋɡʷɑnɑ̃] "to sing". These various changes often led to complex alternations, e.g. *sehwanaN [sexʷɑnɑ̃] ('to see'), *sēgun [sɛːɣun] ('they saw', indicative), *sēwīn [sɛːwiːn] ('they saw', subjunctive), which were reanalysed and regularised differently in the various daughter languages.
Vowels
Proto-Germanic had four short vowels[32] five or six long vowels, and at least one "overlong" or "trimoric" vowel. The exact phonetic quality of the vowels is uncertain. All vowels could also be nasalized when word-final.
PIE ə a o merged into PGmc a; PIE ā ō merged into PGmc ō. At the time of the merger, the vowels probably were [ɑ] and [ɑː], or perhaps [ɒ] and [ɒː]. Their timbres then differentiated by raising (and perhaps rounding) the long vowel to [ɔː]. It is known that the raising of ā to ō can not have occurred earlier than the earliest contact between Proto-Germanic speakers and the Romans. This can be verified by the fact that Latin Rōmānī later emerges in Gothic as Rumoneis (that is, Rūmōnīs). It is explained by Ringe that at the time of borrowing, the vowel matching closest in sound to Latin ā was a Proto-Germanic ā-like vowel (which later became ō). And since Proto-Germanic therefore lacked a mid(-high) back vowel, the closest equivalent of Latin ō was Proto-Germanic ū.
A new ā was formed following the shift from ā to ō when intervocalic /j/ was lost in -aja- sequences. It was a rare phoneme, and occurred only in a handful of words, the most notable being the verbs of the third weak class. The agent noun suffix *-ārijaz (Modern English -er) was likely borrowed from Latin around or shortly after this time.
Diphthongs
The following diphthongs are known to have existed in Proto-Germanic:
Short: /au/, /ai/, /eu/, /iu/
Long: /ɔːu/, /ɔːi/, (possibly /ɛːu/, /ɛːi/)
Diphthongs in Proto-Germanic can also be analysed as sequences of a vowel plus an approximant, as was the case in Proto-Indo-European. Note however the change /e/ > /i/ before /i/ or /j/ in the same or following syllable. This removed /ei/ (which became /iː/) but created /iu/ from earlier /eu/.
Overlong vowels
Proto-Germanic had at least one "overlong" vowel ô (possibly also ê).[33] This occurred in the last syllable of a word, and possibly elsewhere as well. ô is distinguished from ō by the fact that reflexes of the former show up as long vowels while reflexes of the latter are short vowels. There has been a great deal of debate over both the nature of this sound in Proto-Germanic and its origin. Older theories claimed that ô and ō were both long but were distinguished in that ô had a circumflex accent while ō had an acute accent,[clarification needed (Stress? Pitch?)] and asserted that this distinction was inherited from PIE. Newer theories claim that ô was greater in length than ō, and originated mostly through the contraction of directly adjacent vowels, e.g. plural *wulfôz "wolves" < pre-Germanic *wl̥po-es.
ē¹ and ē²
ē² is uncertain as a phoneme, and only reconstructed from a small number of words; it is posited by the comparative method because whereas all provable instances of inherited (PIE) *ē (PGmc. *ē¹) are distributed in Gothic as ē and the other Germanic languages as *ā,[34] all the Germanic languages agree on some occasions of ē (e.g., Got./OE/ON hēr "here" < PGmc. *hē²r). Gothic makes no orthographic and therefore presumably no phonetic distinction between ē¹ and ē², but the existence of two Proto-Germanic long e-like phonemes is supported by the existence of two e-like Elder Futhark runes, Ehwaz and Eihwaz.
Krahe treats ē² (secondary ē) as identical with ī. It probably continues PIE ēi, and it may have been in the process of transition from a diphthong to a long simple vowel in the Proto-Germanic period. Lehmann lists the following origins for ē²: [35]
ēi - Old High German fiara (side).
The preterite of class VII strong verbs with ai, al or an plus a consonant, or ē¹.
iz - Old English mēd, Old High German miata (reward) versus Ancient Greek μισθός (misthos).
Certain pronomial forms, e.g. Old English hēr (here).
Words borrowed from Latin ē or e in the root syllable.