There are three methods of getting your computer or other device to speak for you. The first is to record yourself saying words, phrases, or sentences you want to have available. There is no special software needed for this, just the recorder that came with your computer and a microphone. You can record as many as you want and add more at any time. You will want to organize the individual recordings in a folder with subfolders for different topics such as "position". Just record a single message such as "Move my legs please", name it "Move legs" and save it in the topic folder. Now you can play the recordings on any media player on any computer or device.
The second method is to create your own computer voice using an internet site such as Model Talker. To do this you have to record yourself reading the paragraphs they send you. This has to be done early on while your voice is still good. It takes a lot of time, a quiet room, and a good quality microphone on your computer. Your recordings will then be computer analyzed and turned into a program that can read anything on your computer in your voice. It won't be perfect but it will be you speaking! To use your voice, you will need to be able to type the words you want spoken. For an ALS patient this who can't use the keyboard, it will require typing with an onscreen keyboard or a program such as Dasher (free and terrific!) or eye gaze type of equipment such as Tobii.
The third method is to use a text to speech program. Again you have to give the program something to read so typing in somehow is necessary. Some programs can speak what you have typed as soon as you stop typing, others you click to have it read. Most programs give you a choice of voices to use -- Male, female, and a few choices of each. The voices that come with the programs are not top of the line but are understandable. If you want a better quality voice, smoother, less computer sounding , most text to speach programs let you add other voices that you purchase. You get to hear short clips of voices before you buy.