 Check your own pronunciation. Do you pronounce these words over here with a sharp initial S and say safari, sex, single, and software? Or is your version a pretty German one and you say something like safari, sex, single, and software? Well, if the latter is the case, I think I can do something for you. The problem is a distributional one. Both English and German have a voiceless alveolar fricative as an alofone of the phoneme S, as an English kiss and its German equivalent kuss. English words like sex, however, where we have an initial S are realized with a voiced alofone in German and thus often come out as sex, so the initial S in German is generally voiced. How can we solve that problem and achieve a voiceless variant even in this position? Well, the trick is quite simple. We use those initial S contexts where even Germans use a voiceless S and such contexts exist, namely those where a consonant follows the S. Here are two parallel examples, skeleton and German skillet and Slovenia and the German counterpart Slovenian. In these cases, even Germans use an initial voiceless alveolar fricative. So once we are aware of this fact, present-day English words like snake, slope or swallow must be considered relatively uncomplicated as far as the initial S is concerned. The only thing is we have to transfer this awareness to contexts where the initial S is followed by a vowel. And if this still doesn't help, well, in such cases I suggest to put a short glottal stop, which is of course voiceless in front of each word when you practice it in isolation. So you get a safari, a sex, a single software. Well, and that's almost identical with the English counterpart. Okay, I hope this helps and you will no longer find it difficult to produce present-day English words with an initial S.