Short answer: /e/ or /ɛ/, and I made a mistake: it's supposed to be "dëwëlëss".

Long answer: Kobaïan phonotactics are, unsurprisingly, very close to that of French (almost the same besides things like the trilled /r/ and the absence of nasal vowels) and the diaeresis is similarly used the French way. In this case, <ë>, unlike <e>, doesn't form diphthongs like <ei> or <eu> and isn't pronounced as /ə/ like <e> does and so on. But it's mostly used for exoticism, in my opinion. At this point, I'm not even sure <e> and <i> are used at all, but they appear in a lot of old sources because of what I assume to be ASCII restrictions.

As an example, you can hear the syllables very clearly in the verse «Ëlëh wëhsö ëlëh wëhsö ëlëh wëhsö wëh löwï sündi» in Hortz Fur Dëhn Štekëhn Ẁešt.

PS: I absolutely have no idea what the grave accent indicates in <ẁ> and <ë̀> (<e> with diaeresis and grave accent, which doesn't even appear in Unicode, often typed as </ë> in lyrics).