Speaking to Iraqi fighters, he said in his fifth statement this year and his second in two days that the Islamic community was depending on them.
The tape is meant to show that bin Laden still has power over al Qaeda, former CIA analyst Michael Scheuer told CBS News correspondent Joie Chen.
"It is meant to show 'I'm still here, I'm still in charge of this organization and in fact our organization, our movement is winning,'" Scheuer said.
In the tape, bin Laden says to the Somalis that "Your Muslim nation is looking for you and praying for your victory. You are their hope after God. You are God's trusted soldiers who will liberate the ummah (nation) from the serfdom of the crusaders in our countries."
He called on all Somalis to support the Islamic Courts, which he said were building an Islamic state in the Horn of Africa.
"You have no other means for salvation unless you commit to Islam, put your hands in the hands of the Islamic Courts to build an Islamic state in Somalia," he said.
Bin Laden lashed out at the president of Somalia's secular interim government, Abdullahi Yusuf, calling him a "traitor" and a "renegade."
He warned leaders of Islamic countries against sending troops to Somalia.
"We pledge that we will fight your soldiers on the land of Somalia and we will fight you on your own land if you dispatch troops to Somalia."
The Islamic Courts group took over most of Somalia in June.
Al Qaeda's No. 1 endorsed Abu Hamza al-Muhajer as the new leader for his terror network in Iraq. He urged him to step up "the struggle" to transform Iraq into the center of an Islamic Caliphate.
"I pray to God to make him the best successor to the best predecessor."
He urged militants in Iraq to continue their fight.
"Stay steadfast and don't leave Baghdad, otherwise all the capitals in the region will fall to the crusaders."
Bin Laden also called on leaders of al Qaeda in Iraq to continue to participate in the Mujahideen Shura Council, an umbrella organization grouping Islamist militants fighting U.S. troops and the Iraqi government.
At just over 19 minutes in length, the audio recording it was accompanied by a photo of bin Laden above the logo and name Al-Sahab, al Qaeda's media production wing.
A day before, bin Laden paid tribute to slain leader of al Qaeda in Iraq, Abu Musab al-Zarqawi in another audio tape.
Bin Laden called Zarqawi both a "prince" and a "lion of the holy war" in his tribute released on Friday, CBS News correspondent Richard Roth reports. Bin Laden also said the former al Qaeda in Iraq leader had been under orders to kill Iraqis who supported U.S. forces in the country.
Bin Laden also called on President Bush to release Zarqawi's body and said Jordan should allow the slain terrorist to be buried in his homeland.
Although no new video of the terrorist leader has been seen since October 2004, these were the fourth and fifth audio messages put out this year by bin Laden.
U.S. officials said they believed the voice on Friday's tape to be bin Laden, but have not commented on the new recording.