Always pronounced as voiceless English “s”. As pointed out by Tolkien, it is never pronounced [z] (LotR/1114) as with English “bees”. As a single consonant s appears mostly at the beginning of words, since usually intervocalic [s] became [h] in (Old) Sindarin and then this [h] vanished. Where it survives in the middle of words, it appears primarily in consonant clusters, for example double ss in Lossoth “Snowmen”. The same was true at the end of words, except that late in Sindarin’s phonetic development, [ss] shortened at the end of polysyllables (LotR/1115), for example modern Caradhras “Redhorn” from more archaic †Caran-rass.

In theory, long -ss survives at the end of monosyllables, but Tolkien was rather inconsistent in his presentation of such words, sometimes writing lass or noss and other times las or nos. Helge Fauskanger examined this question at length in his article To SS or not to SS: a Gollumish Problem. His ultimate suggestion was to retain the long -ss as the more common spelling for monosyllables (reducing to -s in polysyllables), and many (Neo) Sindarin writers follow this convention.

However, things are not quite so simple in terms of pronunciation. Of note are plural forms like lais “leaves” and rais “horns” from lass and rass. Ordinarily a final consonant cluster prevents i-intrusion in Sindarin plurals: compare nern “tales” plural of narn versus bair “homes” plural of bâr. However, it seems final ss was either a “weak” cluster that allowed i-intrusion or (more likely) the actual pronunciation was [s] early enough for plurals with ai to form.

Thus I would recommend spelling monosyllables with -ss but pronouncing them with -s. I would further recommend (mirroring Fauskanger) spelling plurals of words ending in -ass with single s, as with lais, rais above, since a diphthong before a cluster (like **laiss) would appear strange. For plurals of monosyllables ending in -ss with other vowels, however, I’d retain the double-s in the plural, such as *biss “wives” plural of bess.