I uploaded new builds last night to make a few minor fixes and changes:

 Fixed: 

• Challenge Mode music stopping
• power-up anims are now in sync
• Resume button not working in pause menu

Some updates:

• Letter weighting now takes into account what letters are on the board instead of just base letter weights — this should reduce the pileup in Es and As that was happening before. Please let me know what you think!
• Swap power-ups now fill faster -- this should make it more fun now that the letter weighting is a bit better. 
• Fastest spawn time for blocks in Challenge mode is now 0.5s -- this is because I'm now unable to lose the game at 1.5s so I wanted more challenge. :)

Enjoy!