Musiala hits hat-trick as Bayern cruise into DFB Pokal last 16, Union Berlin stunned
Harry Kane set Musiala's first up just two minutes in, finding him 10 metres out from goal for the Germany international to hammer home.
Musiala tapped in a rebound from a Kane header to score his second. Replays showed the England-raised Germany forward was offside, but there is no VAR in the German Cup until the next round.
Former Manchester City winger Leroy Sane made it three in the shadows of half-time and Musiala then pounced on a loose ball in the box to score his third, and Bayern's fourth, just before the break to bring up his first hat-trick.
"Even without his goals, he can make the difference," Joshua Kimmich said of Musiala.
The most successful team in German Cup history - Bayern have 20 titles compared to next best Werder Bremen's six - the German giants have struggled in recent years.
Bayern were eliminated at this stage in four of the past five seasons, including by third-division Saarbruecken last season.
Third-division Arminia Bielefeld beat Union Berlin 2-0 at home to become the only third-division side in the hat for the last 16.
A brace from Jakob Lemmer was not enough for the other remaining third-division side, Dynamo Dresden, who lost 3-2 to Darmstadt in extra time, with Aleksandar Vukotic, Tobias Kempe and Isac Lidberg getting the goals.
Earlier, Eintracht Frankfurt forward Omar Marmoush kept up his stellar recent form, scoring the winner in a 2-1 defeat of Borussia Monchengladbach.
Frankfurt were reduced to 10 men when Arthur Theate was sent off for a handball outside the box just 15 minutes in, his second straight red card.
French forward Hugo Ekitike put the five-time winners in front in first-half stoppage time but Gladbach's Ko Itakura levelled things up at the start of the second.
With 20 minutes remaining, Marmoush put the undermanned Eagles back in front, his 11th goal in 13 matches in all competitions this season.
"It was a lot of fun - even with 10 men on the pitch we worked hard."
The Egyptian celebrated with a Halloween mask, telling Sky Germany "I don't actually watch horror movies but I found it in the dressing room and said 'I'll take that with me'."
A Matthias Ginter goal and a penalty from Vincenzo Grifo, both in the first half, took Freiburg to a 2-1 home win over three-time winners Hamburg, who scored through Jonas Meffert in the second half.
Elsewhere, Hertha Berlin beat Heidenheim 2-1 at home thanks to goals from Derry Scherhant and Michael Cuisance. Heidenheim pulled one back through Stefan Schimmer in the 89th minute.
Bremen beat Paderborn 1-0 with Marvin Ducksch scoring the only goal, while Hoffenheim beat the Miroslav Klose-coached Nurnberg 2-1.
On Tuesday, Borussia Dortmund were beaten 1-0 in extra-time away at Wolfsburg, while RB Leipzig and defending champions Bayer Leverkusen had comfortable wins.