Here are your answers. This may be long, so be patient.
1. Is there someway to get a Dectalk-version working with newer JAWS-versions? Does it exist for SAPI 5 or something?
As Jack stated, no, there is no longer a way to get DECtalk Access32 version 4.41 working with newer versions of JAWS, through software, that is. Hardware versions of DECtalk like the USB will work, don't know about the Express as I don't own any of these units myself. Wish I did, but alas, that wasn't meant to be. That said, if you want that classic DECtalk sound we all know and love, go with DECtalk for NVDA as that still works. There IS a SAPI5 version of DECtalk floating around, but it's based on DECtalk 4.6.4, which, it's sort of the classic DECtalk sound, but not exactly. I wouldn't advise you get it, unless you want DECtalk SAPI5 so badly and you don't care how it sounds.
2. I know Dectalk can sings, because I've found some files with Dectalk-songs, but how to do it?
Again, just like Jack stated, DECtalk singing involves assigning phonemes to pitches (from 1 to 37), or in hertz. As for guides, many folks have made guides available on making DECtalk sing. One such fellow is Snoopi Botten, who I guess you could say is a well-known DECtalk user.
3. I've also found a guide to make Eloquence singing, but it doesn't work for me on that Eloquence build in JAWS. What do I need to get it working?
On some guides, the way of making Eloquence sing is the same, but one difference across versions is the way you enter the commands to make Eloquence sing. For example, on JAWS and NVDA, it uses the backquote symbol, not to be confused with the grave accent symbol located to the left of the 1 key on the keyboard, but a different one. Don't remember how to enter it as I don't use Eloquence with NVDA though, so if someone could tell how to enter it, that would be nice. On the Android version, as well as the Eloquence speak window found on grossgang.com/tts/ the symbol to use is the grave accent, which is located to the left of the 1 key, so it would look something like this.
`vb65 `vs140 This is a test.
4. I've heard some funny Microsoft Sam things, but I think, they sound different. Do there exist various versions of Microsoft Sam sounding different?
There are two versions of Microsoft Sam that sound different in the their inflection and how they say things. This information also applies to Mike and Mary.
The first version, sometimes called Speakonian Microsoft Sam or SAPI4 Microsoft Sam, was bundled with Windows 2000. You can get this version working on newer versions of Windows by installing the SAPI4 runtime and the Microsoft TTS engine version 4.0, and this will get you that version of Sam. The only drawback to this is that you can't use it with many things. The only things you can use it with, as far as I know, are screen readers, TTS programs like Balabolka, and many old or early versions of software, such as old versions of programs from APH. The reason why it's called Speakonian Microsoft Sam is because the program Speakonia only can use SAPI4 voices, meaning that you can use Microsoft Sam in that program as well. This version is very popular on YouTube. Emphasis on very popular.
The second version, called SAPI5 Microsoft Sam or Natural Microsoft Sam by some, is the version that was bundled with Windows XP. You'll rarely find this version used on YouTube, if at all. You can get this version of Sam working on anything newer than XP if you install WinXP_TTS_Voices, which you can get from grossgang.com/tts/ under the SAPI voices directory.
5. Can I, in some way, make a speech synthesizer change it's rate within a line on text?
Yes, you can. Depending on the synth in question, the way of accessing the rate command is different. For example, Eloquence uses the tag `vs, MacinTalk uses the tag [[rate xx]], and so on. Keep in mind that, due to how screen readers handle punctuation, these commands may not work without modifications.