I have a song that I'm trying to make into a MIDI, and I have most of it down, but there's small part at the end that I can't pick out, no matter what I do. The problem is that it's strings playing very fast, short notes, and it's in the background of the piece, so I can't always hear it completely over the main instruments. Has anyone else run into this predicament and/or have any tips for how I can pick out the less-audible sounds so I can complete my transcription.
You can use logic to help you approximate the part, or even get really, really close.
First, you say fast notes, but what exactly are the rhythms? Even 16ths? Is there an ostinato? See if you can transcribe the rhythms first before attempting pitches.
Second, let’s say you get those rhythms down and are now looking for notes: what is the tonality? Think logically here: very likely the notes aren’t random, and since they’re backing material, it means that they’re working in combination with the foreground material, which you’ve already presumably transcribed. Is there a chord progression? Can you put your rhythms to the chord progression? Can you make out the top and bottom lines for those chords, using them to figure out each chord’s voicing?
Answers to these questions will help guide your practice.
The final studio mix is often done in a way to blend the string tracks together so that they have a unified sound. With that version of the recording you could try to slow it down with software such as Amazing Slow Downder or Transcribe.
You could also look for backing tracks; very often you can isolate the different tracks so you could get an isolated guitar track.
If those options fail, you'll be left with finding other trnscriptions to compare notes.
If you can find the song on YouTube, and you're scared of downloading, there's always slowing down the YouTube video to 75% or 50% speed and trying to pick out notes or match rhythms this way. Note that, at the time of this answer, YouTube's 25% speed setting does not produce sound.