Newgrounds groups like the Clock Crew and the Lock Legion use a program called Speakonia.
If you have a Mac emulator set up with System 7.5.5, you have access to a couple dozen voices that are much nicer than Microsoft Sam.
If you can get a copy of Mini vMac, the vMac ROM, and a copy of MacinTalk you can use a Mac speech synthesis program written in 1983... that sounds better than Microsoft Sam.

(I have a boot disk image ready to go, which is redistributable without breaking rules, but if you want to use it you need the Mac BIOS image, which I can't give you. You'd have to find it yourself.

)
If you use the latter two options you will need some sort of audio recording software that can record what the speakers produce. Audacity set to "Stereo Mix" works well, and can record well past Microsoft sound recorder's 60 second limit.