How are sounds used to create mood?
The main sounds that I teach to students are
1. sibilant /s/ /sh/ /ch/ /x/
2. plosive /b/ /p/ /t/ /d/
3. liquid /l/
4. (sometimes) fricative, voiced /th/ ‘the’ /v/ and unvoiced /th/ ‘theatre’ /f/
5. (rarely) nasal /m/ /n/

Should we always comment on sounds in writing?
No. Sounds are everywhere. Some are deliberate, but most are accidental. Only a few will help prove your point about the theme or meaning. Choose examples with care. Two per poem would be enough. Use your own judgement.

Sibilant /s/ sounds. This can be written <s> <ss> or <c> as in ‘ice’. <sh> <dg> <x> <ks> or <ch>
WARNING: This is actually one of the most common sounds in English, so be careful to make sure the writer is deliberately using it for effect.

Why is it so common? In English, plurals end in -s or -es, a lot of verbs end in -s for the present tense, and it appears in very common words like ‘is’.

What is the effect of the sibilant /s/ sound?
The effect depends on context, and also the meanings of the words around it.
It can be: 
– hissing or insidious

You can also call this ‘onomatopoeia’.
How to write about it: ‘the sibilant ‘s’ creates an onomatopoeic, sinister effect’ OR ‘the onomatopoeic sibilant sounds create a sinister effect’.

More examples:
How to write about it: e.g. The sibilant sounds in ‘softly, sweetly, sickly’ creates a soft, gentle mood, which turns sinister on ‘sickly’ as the sounds flow across the line. The unusual shift in mood within the same, sibilant sound, creates a disturbing effect.

Plosive /b/ /p/ /t/ /d/ sounds create an abrupt, sharp, sometimes shocking effect. Look for plosives blended with sibilants or liquids – as a short, sharp shock after the softer mood OR, where both are interlaced (sib/plo/sib/plo/sib/plo) think about which feels stronger – is it a juddering effect, stuttering (be creative with your interpretation: what does it make you think of?)
How to write about it:
At the end of the stanza the places are listed as if an incantation, repeating the monosyllabic plosives : ‘Belfast. Beirut. Phom Penh.’ 

Liquid  /l/ this can flow, creating a sense of quick, light movement – or of water – ‘light slipped down the lee of the hill’, or sound thick, heavy when combined with dull sounds – as in ‘ladle’, ‘paddle’ and ‘paddle’.

Fricatives /f/ /v/ /th/ /th/
These are divided into voiced (hard) and voiceless (soft). It’s fun to notice the connection: the <f> in ‘knife’, is soft, and becomes hard in the plural ‘knives’. The same thing happens in ‘loaf’ and ‘loaves’.
Voiceless fricatives can create an airy effect.
e.g. In the poem ‘Flag’ by John Agard, he repeats voiceless fricatives at the start of the first two stanzas: ‘flag’, ‘fluttering’ ‘unfurled’, to create a free, flowing and airy effect – of freedom.

When commenting on sounds: remember – it’s not just noise! It’s words. Words have meanings. So you need to figure out how the sounds add to (or contrast with) the meaning.
You can’t just take a one size fits all approach.
Sibilants aren’t always gentle. Sometimes they’re sinister.
Sometimes flowing sounds are positive. Other times they may flow too fast as if they’re out of control.
Plosives are sometimes harsh in a bad way, sometimes energetic and bouncy. When combined with liquids, they can sound sensual.

Plosives and Liquids together:
At first, though we’re introduced to the main characters in an empty, Eden-like place of natural beauty of ‘warm’ ‘golden’ beauty, with sensory language like ‘pool’ ‘slopes’ and ‘Gabilan’ that run sensually over liquids (l) and plosives (p,b). Steinbeck’s love of his home landscape is tangible. His use of the present tense ‘the Salinas river drops… and runs deep and green’ give it a timeless, eternal feel, which adds to the Edenic quality.

Sibilants, Voiceless Fricatives and Liquids together: 

The mood is made even more awkward through the rich and sensual sounds, of sibilants, liquids/voiceless fricatives and long syllables in ‘spools‘ and ‘suffering‘.