In German standard pronunciation, there are contrasting /s/ and /z/ phonemes; for instance, Busen [ˈbuːzn̩] and Bußen [ˈbuːsn̩] differ only in the voicedness of the s (a so-called minimal pair). This difference is, however, neutralized both at the start and at the end of words: At the start of words, only a voiced [z] may appear; at the end of words, only a voiceless [s] is possible. Exceptions are sometimes made for loanwords: Skala [ˈskaːla], Sex [sɛks] (in contrast to the numeral sechs [zɛks]). As with other cases of Auslautverhärtung, intentionally pronouncing a voiced [z] at the end of a word is difficult for many German native speakers, contributing to the German accent in foreign languages.
Within words, the different s sounds are not always clearly marked in the spelling. A ß always corresponds to a voiceless [s] and a ss most of the time; but an s may stand for [z] or [s]:
Felsen [ˈfɛlzn̩], Gläser [ˈɡlɛːzɐ], Fasten [ˈfastn̩], Obst [oːpst]
The right pronunciation can sometimes be told from the adjacent sounds, though not always.
All of the above concerned the standard pronunciation. In reality, there is huge regional and individual variation in the pronunciation of the s sounds. In Germany’s southern half, [s] prevails for word-initial s, but even in the northern parts, far from everyone will follow the standard pronunciation. A speech and language therapist also told me once that [z] is considered the most difficult sound in German, causing difficulties for many children when learning the language. Some of them may give up on trying to produce [z] at all.
The speaker who provided your sample seems to like the voiceless [s]. :-) He not only says [ˈsamsa] (Samsa), but also [ˈkʁiːsə] (Krise) and [bəˈsɪçtɪɡʊŋ] (Besichtigung); though there are examples such as simultan where he in fact does say [z]. According to his profile, he is from Franconia in the southern half of Germany and now lives in Sweden (Swedish does not have a voiced [z] at all); perhaps the non-standard s pronunciation has to do with this background.
Now for the pronunciation of Samsa. As already remarked by Toscho, it is a name, and names do not always follow the standard rules; as it is an invented name, however, we may attempt an informed guess. The relevant rules are these:
- As stated above, s at the start of a word is voiced.
- In the combinations ls, ms, ns, rs between vowels, the s is voiced. (Bremse [ˈbʁɛmzə])
Thus, the standard pronunciation should indeed be [ˈzamza].