I'd like to learn how to sing songs. I have sheet music with 4 part harmony, but I don't really know how to read notes that well. Are there any tools that can sort of read sheet music for me? Like if I could tell it to play the bass or tenor part.
is the best tool for doing what you want.
Put the sheet music on a scanner, feed the scans into PhotoScore, and it uses OCR (Optical Character Recognition) to convert it to digital music notation which it can then play back using software MIDI instrument sounds. It's not perfect, and to use it effectively, you need to know how to manually correct the reading mistakes that it makes. It has a rather steep learning curve.
While it will work stand-alone, it is designed to be used in conjunction with Sibelius. You export PhotoScore's results to Sibelius where you can do a lot more editing and more sophisticated audio playback of the music as well.
If you buy Sibelius (student discounts are available) it comes with a limited version of PhotoScore. You can then purchase an upgrade to the full version of PhotoScore and save some money.
All this is expensive and has a serious learning curve. You will have to spend hours reading the owners manuals and working through the tutorials before you can get any useful results. But this the best system I can recommend. I have used both of these programs for several years.
Yet another established OCR-for-sheet-music product is SharpEye. I have no experience with it.
Since you are only working with some simple 4-part chorales and you are a beginner, I think it's more practical to go with the suggestions of others here and get an inexpensive notation program and learn to manually enter the notes from the printed sheet music into the computer for musical playback and analysis. PhotoScore and OCR are probably overkill for your situation.
MuseScore, which is free, or the inexpensive commercial programs Sibelius First and Finale Songwriter would be good places to start. Apple GarageBand can do simple notation and excellent musical playback and comes free if you have a Mac.
I would avoid Noteworthy Composer for Windows because, while it is inexpensive, it is non-standard, does not use MusicXML, can't interface with anything else, and by the way, does not produce professional-quality sheet music. LilyPond is amazing and free but extremely hard to use, especially for someone with no formal training as to how sheet music is supposed to be properly constructed. LilyPond is best left to musical experts.
I know choirs whose members use Finale Reader which is a free program that can display and play music from Finale or MusicXML files.
Obviously it requires the piece you are learning to be in Finale or MusicXML.
Leaving a more generic answer on an old question: This doesn't have to be a request for a specific piece of software. The "tool" that makes this possible is MIDI, the computer-based way of representing music. There are many applications, as noted in other answers, that can generate a midi file optically from printed music, but if you can find a pre-existing midi file, it's even easier to open it with any of dozens of applications, and then be able to play back one voice at a time. (A word of warning, though, there are plenty of "messy" midi files out there that might be confusingly formatted or notated. It's just a file format, and there's no more guarantee of quality than there is that a Microsoft Word document contains good writing.)