Some questions

[avra]

Huh? eSpeak seems to be GPL licensed. If you just execute the application (i.e. don't link to it) that would be fine in a commercial application.
You'd only have to give out the source of your commercial app if you linked to eSpeak or if you use code from it.

Or is there something I've missed? (Maybe license of the voices/languages?)

I would suggest that the choice of tool and library should be guided by the needs of the app and its users. Shelling out to a console app to do text-to-speech seems, well, inappropriate for commercial software, which can't look awkward or amateurish.

How would you know that some app is using command line TTS if you never see console window? Besides, eSpeak also has DLL/SO so with a little work Pascal wrapper can be made around it.

Here is an example using eSpeak in Delphi:
http://www.rolfware.de/delphi/espeak_example.html

[Zoran]

Besides, eSpeak also has DLL/SO so with a little work Pascal wrapper can be made around it.

Non-GPL licenced application is not allowed to link to GPL licenced library.

[avra]

Besides, eSpeak also has DLL/SO so with a little work Pascal wrapper can be made around it.

Non-GPL licenced application is not allowed to link to GPL licenced library.

I would not name calling external DLL from an application a linking operation. That should be allowed, shouldn't it?

[Zoran]

I would not name calling external DLL from an application a linking operation.

What else could "dynamic linking to a library" mean?

That should be allowed, shouldn't it?

If the authors had wanted to allow it, they would not have chosen GPL for their work. They would have chosen LGPL at least. This is what sets those two apart.

[fabienwang]

Zoran, on the link you provided, they talk about the exception of GPL (section 7 of the GPL Licence)

The increasing use of dynamic linkage has implications for software licensing. For example, the GPL linking exception allows programs which do not license themselves under GPL to link to libraries licensed under GPL without thereby becoming subject to GPL requirements.

[Zoran]

Zoran, on the link you provided, they talk about the exception of GPL (section 7 of the GPL Licence)

The increasing use of dynamic linkage has implications for software licensing. For example, the GPL linking exception allows programs which do not license themselves under GPL to link to libraries licensed under GPL without thereby becoming subject to GPL requirements.

They only mention it there, but that is quite another matter. By adding exception to GPL you create new licence, diferrent from original GPL (actually, LGPL is just that). Read more about possibility of adding exceptions to GPL: http://en.wikipedia.org/wiki/GPL_linking_exception

[avra]

After reading http://en.wikipedia.org/wiki/GNU_General_Public_License, especially chapters:
1. Point of view: dynamically and static-linking violates GPL
2. Point of view: static linking violates GPL but unclear as of dynamic linking
3. Point of view: linking is irrelevant
all I can say is that I will avoid calling external GPL lib in my commercial applications. It is not quite clear, but I will stay away just to keep on the safe side. Lucky me, I haven't used any so far, but that is just pure luck. It seams that calling external GPL application via command line parameters is safe.