I transposed a piece of music but the result is too quiet. How do I redo with louder volume?
Effect → Amplify. 
Note that there is a “digital maximum” of 0dBFS and if you go “louder”, you’ll get clipping (distorted flat-topped waves). But the peaks don’t correlate well with loudness so you can have a quiet-sounding track that’s normalized (AKA maximized) to have 0dB peaks.
If that’s not loud enough you can use dynamic compression (The Compressor effect) to boost the overall loudness (but this is a non-linear effect that changes the “character” of the sound).
People new to recording always have the same complaint: How come this isn’t as loud as the music on iTunes or other service or the CD I bought? Those performers have been through the “loudness war” where special distortion is added to make the sound louder, denser and more forward without actually causing harsh sound damage. It’s a good trick.
Effect > Compressor is one way to do that, but a cousin to that tool is Chris’s Compressor.
http://theaudacitytopodcast.com/chriss-dynamic-compressor-plugin-for-audacity/
This is a sample of blue waves with no compression, default settings and then again with the Compression Ratio setting increased from 0.5 to 0.77. Note the blue waves are getting overall higher and more dense without going over 100%.
Koz